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Summary of Non-Performance Based Restricted Stock Units (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Award, Fair Value [Abstract]      
Shares reserved for future issuance 3,000,000    
Restricted stock units (RSUs)      
Share-based Compensation Arrangement by Award [Line Items]      
Exercise price $ 0.00    
Share-based Compensation Arrangement by Award [Abstract]      
Balance at Beginning of Period 133,853 107,814 102,786
Granted 76,586 62,453 85,184
Vested (25,803) (29,010) (75,687)
Forfeited (9,536) (7,404) (4,469)
Balance at End of Period 175,100 133,853 107,814
Share-based Compensation Arrangement by Award, Fair Value [Abstract]      
Shares outstanding at the beginning of the period, weighted average grant date fair value $ 20.13 $ 16.48 $ 15.43
Granted, weighted average grant date fair value 36.83 23.66 17.40
Vested, weighted average grant date fair value 18.45 14.73 16.12
Forfeited, weighted average grant date fair value 20.48 17.97 15.85
Shares outstanding at the end of the period, weighted average grant date fair value $ 27.66 $ 20.13 $ 16.48
Share-based compensation expense $ 1,300,000 $ 800,000 $ 500,000
Share-based compensation not yet recognized $ 2,900,000    
Share-based compensation not yet recognized, period for recognition 2 years 1 month    
Restricted stock units (RSUs) | Minimum      
Share-based Compensation Arrangement by Award, Fair Value [Abstract]      
Performance vesting period 1 year    
Restricted stock units (RSUs) | Maximum      
Share-based Compensation Arrangement by Award, Fair Value [Abstract]      
Performance vesting period 3 years    
Senior Management and Other Officers | Restricted stock units (RSUs)      
Share-based Compensation Arrangement by Award [Abstract]      
Granted 27,764 21,905 56,611
Management | Restricted stock units (RSUs)      
Share-based Compensation Arrangement by Award [Abstract]      
Granted 18,366 22,059  
Certain Employees And Executives | Restricted stock units (RSUs)      
Share-based Compensation Arrangement by Award [Abstract]      
Granted 30,456 18,489 28,573