Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity, Stock Options and Net Income per Share - Non-employee Director Non-vested Shares (Details)

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Shareholders' Equity, Stock Options and Net Income per Share - Non-employee Director Non-vested Shares (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
1 Months Ended 12 Months Ended
May 31, 2016
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Non-Vested Shares [Line Items]        
Share-based compensation   $ 11,907 $ 10,549 $ 8,103
Nonemployee Director Nonvested Shares Granted [Member]        
Non-Vested Shares [Line Items]        
Additional shares authorized available for grant 160      
Number of shares authorized under the plan   360    
Shares available for grant   116    
Outstanding at beginning of year (shares)   15 11 16
Granted (shares)   16 16 14
Vested (shares)   (15) (12) (16)
Forfeited (shares)       (3)
Outstanding at end of year (shares)   16 15 11
Weighted average grant date fair value of non-vested shares at beginning of year   $ 59 $ 52 $ 44
Weighted Average Grant Date Fair Value   62 59 52
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value   64 52 44
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value       49
Weighted average grant date fair value non-vested shares at end of year   $ 62 $ 59 $ 52
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Total Fair Value   $ 990 $ 920 $ 742
Grant date fair value of shares that vested during the year   970 615 809
Share-based compensation   970 775 608
Tax benefit related to share-based compensation expense   244 $ 195 $ 216
Unrecognized compensation cost   $ 368    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition   4 months 24 days