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Employee Benefits - Summary of Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summary of option activity [Rollforward]      
Number of share options, Outstanding, Beginning balance (in shares) 747,333 451,999 554,998
Number of share options, Granted (in shares) 562,500 395,000 70,000
Number of share options, Exercised (in shares) (222,676) (78,666) (150,999)
Number of share options, Forfeited or expired (in shares) (20,000) (21,000) (22,000)
Number of share options, Outstanding, Ending balance (in shares) 1,067,157 747,333 451,999
Number of share options, Exercisable (in shares) 182,350 184,671 167,999
Summary of option activity, weighted average exercise price [Roll Forward]      
Weighted Average Exercise Price, Outstanding, Beginning balance (in dollars per share) $ 15.89 $ 13.41 $ 12.12
Weighted Average Exercise Price, Granted (in dollars per share) 23.38 17.87 16.82
Weighted Average Exercise Price, Exercised (in dollars per share) 13.86 11.83 10.18
Weighted Average Exercise Price, Forfeited or expired (in dollars per share) 23.45 14.88 13.94
Weighted Average Exercise Price, Outstanding, Ending balance (in dollars per share) 20.12 15.89 13.41
Weighted Average Exercise Price, Exercisable (in dollars per share) $ 15.95 $ 12.68 $ 11.03
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]      
Aggregate Intrinsic Value, Exercised $ 1,937 $ 735 $ 1,557
Aggregate Intrinsic Value, Ending balance 2,648    
Aggregate Intrinsic Value, Options exercisable at end of year $ 1,066
Share-based Compensation Arrangement, Weighted Average Remaining Contractual Life [Abstract]      
Weighted Average Remaining Contractual Life, Outstanding, end of year (Years) 3 years 8 months 1 day    
Weighted Average Remaining Contractual Life, Options exercisable at end of year (Years) 2 years 5 months 14 days    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest [Abstract]      
Number of shares options, Vested and Expected to Vest (in shares) 1,067,157 747,333 437,799
Weighted Average Exercise Price, Vested and Expected to Vest (in dollars per share) $ 20.12 $ 15.89 $ 13.36
Aggregate Intrinsic Value, Vested ans Expected to Vest (in shares) $ 2,648    
Weighted Average Remaining Contractual Life (Years) 3 years 8 months 1 day