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)

v3.8.0.1
Shareholders' Equity, Stock Options and Net Income per Share - Non-employee Director Non-vested Shares (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
May 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Non-Vested Shares [Line Items]        
Share-based compensation   $ 8,103 $ 8,334 $ 7,486
Nonemployee Director Nonvested Shares Granted [Member]        
Non-Vested Shares [Line Items]        
Non-vested share vesting period   1 year    
Outstanding at beginning of year (shares)   16,000 15,000 15,000
Granted (shares)   14,000 16,000 14,000
Vested (shares)   (16,000) (15,000) (14,000)
Forfeited (shares)   (3,000)    
Outstanding at end of year (shares)   11,000 16,000 15,000
Weighted average grant date fair value of non-vested shares at beginning of year   $ 44 $ 51 $ 44
Weighted Average Grant Date Fair Value   52 44 51
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value   44 51 43
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   $ 52 $ 44 $ 51
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Total Fair Value   $ 742 $ 688 $ 740
Grant date fair value of shares that vested during the year   809 639 727
Share-based compensation   608 728 661
Tax benefit related to share-based compensation expense   216 $ 263 $ 259
Unrecognized compensation cost   $ 215    
Additional shares authorized available for grant 160,000      
Number of shares authorized under the plan   360,000    
Shares available for grant   148,019    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition   4 months 11 days