Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
Shareholders' Equity, Stock Options and Net Income per Share - Employee Activity - Performance Shares (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Employee Activity - Performance Shares [Line Items]      
Share-based compensation $ 8,103 $ 8,334 $ 7,486
Key Employee Performance Share Based Plan [Member]      
Employee Activity - Performance Shares [Line Items]      
Share-based awards, vesting period 3 years    
Minimum percentage of peer group by which Company share price must outperform before incremental performance shares are issued 25.00%    
Percentage of Peer Group By Which Company Share Price Must Outperform Before Maximum Incremental Shares Are Issued 90.00%    
Outstanding at beginning of year (shares) 80 77 74
Granted (shares) 27 29 27
Additional shares awarded based on performance 0 7 0
Vested (shares) 0 (33) (24)
Forfeited (shares) (38)    
Outstanding at end of year (shares) 69 80 77
Weighted average grant date fair value of non-vested shares at beginning of year $ 55 $ 52 $ 44
Weighted average fair value of performance shares 56 49 67
Stock Issued During Period, Weighted Average Grant Date Fair Value 0 40 0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value 0 40 45
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value 51    
Weighted average grant date fair value non-vested shares at end of year $ 58 $ 55 $ 52
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Total Fair Value $ 3,980 $ 4,373 $ 4,016
Share-based compensation 1,045 1,447 1,308
Tax benefit related to share-based compensation expense 371 $ 537 $ 512
Unrecognized compensation cost $ 1,383    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 8 months 11 days