Share-Based Payments (Tables)
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9 Months Ended | 
Sep. 30, 2019  | 
| Employee Stock Option [Member] | 
 
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | 
 
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| Share-based Compensation, Activity [Table Text Block] | 
 
  
The following tables summarize the Company’s employee stock option activity and related information: 
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Nine months ended September 30, 2019  | 
 
 
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Weighted-  | 
 
 
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Weighted-  | 
 
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Average  | 
 
 
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Average  | 
 
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Aggregate  | 
 
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Remaining  | 
 
 
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Exercise  | 
 
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Intrinsic  | 
 
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Contractual  | 
 
 
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Options  | 
 
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Price  | 
 
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Value  | 
 
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Term  | 
 
Outstanding at December 31, 2018  | 
538  | 
 
  | 
 
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$  | 
51  | 
 
  | 
 
  | 
 
  | 
 
  | 
 
  | 
 
Exercised  | 
(45  | 
)  | 
 
  | 
47  | 
 
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  | 
 
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  | 
 
  | 
 
Forfeited  | 
(8  | 
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  | 
54  | 
 
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Outstanding at September 30, 2019  | 
485  | 
 
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$  | 
52  | 
 
  | 
 
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$  | 
4,638  | 
 
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  | 
4.0  | 
 
Exercisable at September 30, 2019  | 
319  | 
 
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$  | 
48  | 
 
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$  | 
4,163  | 
 
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  | 
3.3  | 
 
  
 
 
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Nine months ended  | 
 
 
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September 30,  2019  | 
 
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September 30,  2018  | 
 
Share-based compensation for options  | 
$  | 
1,209  | 
 
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  | 
$  | 
1,085  | 
 
  | 
 
Tax benefit for option compensation  | 
$  | 
312  | 
 
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$  | 
271  | 
 
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Unrecognized compensation cost for options, net of estimated forfeitures  | 
$  | 
1,878  | 
 
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$  | 
3,621  | 
 
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Weighted average period over which unrecognized compensation will be recognized (years)  | 
1.6  | 
 
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| Employee Non-vested Shares [Member] | 
 
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | 
 
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| Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] | 
The following tables summarize the Company’s employee non-vested share activity and related information:
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Nine months ended September 30, 2019  | 
 
 
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Weighted-  | 
 
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Average  | 
 
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Aggregate  | 
 
 
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Non-vested  | 
 
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Grant Date  | 
 
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Grant Date  | 
 
 
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Shares  | 
 
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Fair Value  | 
 
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Fair Value  | 
 
Outstanding and non-vested at December 31, 2018  | 
315  | 
 
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$  | 
55  | 
 
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Granted  | 
114  | 
 
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  | 
59  | 
 
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Vested  | 
(131  | 
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61  | 
 
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Forfeited  | 
(15  | 
)  | 
 
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56  | 
 
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Outstanding and non-vested at September 30, 2019  | 
283  | 
 
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$  | 
58  | 
 
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$  | 
16,441  | 
 
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Nine months ended  | 
 
 
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September 30,  2019  | 
 
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September 30,  2018  | 
 
Share-based compensation for non-vested shares  | 
$  | 
6,210  | 
 
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$  | 
4,902  | 
 
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Tax benefit for non-vested share compensation  | 
$  | 
1,581  | 
 
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$  | 
1,225  | 
 
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Unrecognized compensation cost for non-vested shares, net of estimated forfeitures  | 
$  | 
10,705  | 
 
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$  | 
13,146  | 
 
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Weighted average period over which unrecognized compensation will be recognized (years)  | 
1.8  | 
 
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| Key Employee Performance Share Based Plan [Member] | 
 
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | 
 
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| Share-based Compensation Arrangements by Share-based Payment Award, Performance-Based Units, Vested and Expected to Vest [Table Text Block] | 
 
The fair value of the performance shares granted based on the three year performance of the Company’s total shareholder return was estimated using a Monte Carlo simulation.  The weighted average assumptions used in the Monte Carlo estimate were as follows:
 
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Nine months ended  | 
 
 
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September 30,  2019  | 
 
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September 30,  2018  | 
 
Expected stock price volatility  | 
23.4  | 
%  | 
 
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24.3  | 
%  | 
 
Weighted average risk-free interest rate  | 
2.5  | 
%  | 
 
  | 
2.2  | 
%  | 
 
  
The following tables summarize the Company’s employee performance share activity, assuming median share awards, and related information: 
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Nine months ended September 30, 2019  | 
 
 
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Weighted-  | 
 
  | 
 
  | 
 
 
  | 
 
  | 
 
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Average  | 
 
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Aggregate  | 
 
 
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Performance  | 
 
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Grant Date  | 
 
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Grant Date  | 
 
 
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Shares  | 
 
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Fair Value  | 
 
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Fair Value  | 
 
Outstanding and non-vested at December 31, 2018  | 
65  | 
 
  | 
 
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$  | 
58  | 
 
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  | 
 
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Granted  | 
30  | 
 
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  | 
61  | 
 
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Vested  | 
(23  | 
)  | 
 
  | 
64  | 
 
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Forfeited  | 
(10  | 
)  | 
 
  | 
63  | 
 
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Outstanding and non-vested at September 30, 2019  | 
62  | 
 
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$  | 
62  | 
 
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$  | 
3,870  | 
 
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Nine months ended  | 
 
 
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September 30,  2019  | 
 
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September 30,  2018  | 
 
Share-based compensation for performance shares  | 
$  | 
821  | 
 
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$  | 
953  | 
 
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Tax benefit for performance share compensation  | 
$  | 
212  | 
 
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$  | 
238  | 
 
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Unrecognized compensation cost for performance shares, net of estimated forfeitures  | 
$  | 
1,884  | 
 
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$  | 
1,725  | 
 
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Weighted average period over which unrecognized compensation will be recognized (years)  | 
1.9  | 
 
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| Employee Stock Purchase Plan [Member] | 
 
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | 
 
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| Schedule of Share-based Compensation, Employee Stock Purchase Plan, Activity [Table Text Block] | 
The following table summarizes the Company’s employee stock purchase activity and related information:
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Nine months ended  | 
 
 
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September 30,  2019  | 
 
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September 30,  2018  | 
 
Shares purchased by participants under plan  | 
5  | 
 
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5  | 
 
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Average purchase price  | 
$  | 
49  | 
 
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$  | 
52  | 
 
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Weighted-average fair value of each purchase right under the ESPP granted ¹  | 
$  | 
10  | 
 
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$  | 
7  | 
 
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Share-based compensation for ESPP shares  | 
$  | 
52  | 
 
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$  | 
32  | 
 
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¹ Equal to the discount from the market value of the common stock at the end of each six month purchase period  |  
  
 
 
  
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| Nonemployee Director Nonvested Shares Granted Member | 
 
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | 
 
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| Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] | 
The following tables summarize the Company’s non-employee non-vested share activity and related information:
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  | 
Nine months ended September 30, 2019  | 
 
 
  | 
 
  | 
 
  | 
Weighted-  | 
 
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  | 
 
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Average  | 
 
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Aggregate  | 
 
 
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Non-vested  | 
 
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Grant Date  | 
 
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Grant Date  | 
 
 
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Shares  | 
 
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Fair Value  | 
 
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Fair Value  | 
 
Outstanding and non-vested at December 31, 2018  | 
15  | 
 
  | 
 
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$  | 
59  | 
 
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Granted  | 
15  | 
 
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  | 
62  | 
 
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Vested  | 
(15  | 
)  | 
 
  | 
59  | 
 
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Outstanding and non-vested at September 30, 2019  | 
15  | 
 
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$  | 
62  | 
 
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$  | 
920  | 
 
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Nine months ended  | 
 
 
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September 30,  2019  | 
 
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September 30,  2018  | 
 
Share-based compensation for non-vested shares  | 
$  | 
714  | 
 
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$  | 
553  | 
 
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Tax benefit for non-vested share compensation  | 
$  | 
184  | 
 
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$  | 
138  | 
 
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Unrecognized compensation cost for non-vested shares, net of estimated forfeitures  | 
$  | 
554  | 
 
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$  | 
501  | 
 
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Weighted average period over which unrecognized compensation will be recognized (years)  | 
0.6  | 
 
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