Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity, Stock Options and Net Income per Share - Stock Option Activity (Details)

v3.8.0.1
Shareholders' Equity, Stock Options and Net Income per Share - Stock Option Activity (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation $ 8,103 $ 8,334 $ 7,486
Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at beginning of year (shares) 564 786 1,363
Granted (shares) 128 137 96
Exercise of stock options (in shares) (206) (346) (659)
Forfeited (shares) (46) (13) (14)
Outstanding at end of year (shares) 440 564 786
Exercisable at end of year 226 331 586
Weighted Average Outstanding at beginning of year (per share) $ 41 $ 32 $ 28
Weighted average exercise price for options granted in the period 48 44 50
Weighted average exercise price for options exercised in the period 35 24 26
Weighted average exercise price for options forfeited in the period 46 35 29
Weighted Average Outstanding at end of year (per share) 45 41 32
Weighted average exercise price for options exercisable at year end 42 37 28
Weighted-average fair value of options granted during the year (dollars per share) $ 13 $ 12 $ 15
Aggregate intrinsic value of options exercised $ 3,569 $ 7,803 $ 16,191
Average aggregate intrinsic value for options outstanding 3,387    
Average aggregate instrinsic value for exercisable options 2,259    
Share-based compensation 1,313 1,473 1,386
Tax benefit related to share-based compensation expense 466 $ 546 $ 542
Unrecognized compensation cost $ 1,647    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 9 months 11 days    
Exercise Price Range 36.55 thru 37.14 [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at end of year (shares) 83    
Exercisable at end of year 83    
Weighted Average Outstanding at end of year (per share) $ 36.85    
Weighted average exercise price for options exercisable at year end 36.85    
Range of exercise price, lower range limit 36.55    
Range of exercise price, upper range limit $ 37.14    
Weighted average remaining contractual life 1 year 7 months 11 days    
Exercise Price Range 41.32 thru 44.90 [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at end of year (shares) 159    
Exercisable at end of year 90    
Weighted Average Outstanding at end of year (per share) $ 43.29    
Weighted average exercise price for options exercisable at year end 43.07    
Range of exercise price, lower range limit 41.32    
Range of exercise price, upper range limit $ 44.90    
Weighted average remaining contractual life 4 years 5 months 11 days    
Exercise Price Range 45.34 thru 48.32 [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at end of year (shares) 123    
Exercisable at end of year 10    
Weighted Average Outstanding at end of year (per share) $ 47.73    
Weighted average exercise price for options exercisable at year end 46.44    
Range of exercise price, lower range limit 45.34    
Range of exercise price, upper range limit $ 48.32    
Weighted average remaining contractual life 6 years 11 days    
Exercise Price Range 50.71 thru 53.73 [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at end of year (shares) 70    
Exercisable at end of year 43    
Weighted Average Outstanding at end of year (per share) $ 51.03    
Weighted average exercise price for options exercisable at year end 50.81    
Range of exercise price, lower range limit 50.71    
Range of exercise price, upper range limit $ 53.73    
Weighted average remaining contractual life 4 years 3 months 11 days    
Exercise Price Range 57.18 thru 57.18 [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at end of year (shares) 5    
Exercisable at end of year 0    
Weighted Average Outstanding at end of year (per share) $ 57.18    
Weighted average exercise price for options exercisable at year end 0.00    
Range of exercise price, lower range limit 57.18    
Range of exercise price, upper range limit $ 57.18    
Weighted average remaining contractual life 7 years 11 days    
Exercise Price Range 36.55 thru 57.18 [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at end of year (shares) 440    
Exercisable at end of year 226    
Weighted Average Outstanding at end of year (per share) $ 44.70    
Weighted average exercise price for options exercisable at year end 42.39    
Range of exercise price, lower range limit 36.55    
Range of exercise price, upper range limit $ 57.18    
Weighted average remaining contractual life 3 years 6 months 11 days