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