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Summary of Non-Perfomance Based Restricted Stock Units (Details) - Restricted stock units (RSUs) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Award [Abstract]    
Balance at Beginning of Period 175,100 133,853
Granted 35,048 61,986
Vested (31,021) (10,719)
Forfeited (2,611) (406)
Balance at End of Period 176,516 184,714
Share-based Compensation Arrangement by Award, Fair Value [Abstract]    
Shares outstanding at the beginning of the period, weighted average grant date fair value $ 27.66 $ 20.13
Granted, weighted average grant date fair value 64.60 31.64
Vested, weighted average grant date fair value 30.68 15.17
Forfeited, weighted average grant date fair value 14.75 24.71
Shares outstanding at the end of the period, weighted average grant date fair value $ 37.22 $ 24.27
Share-based compensation expense $ 0.4 $ 0.3
Share-based compensation not yet recognized $ 4.5  
Share-based compensation not yet recognized, period for recognition 2 years 5 months  
Senior Management    
Share-based Compensation Arrangement by Award [Abstract]    
Granted 15,867  
Management    
Share-based Compensation Arrangement by Award [Abstract]    
Granted 11,780  
Certain Employees And Executives    
Share-based Compensation Arrangement by Award [Abstract]    
Granted 7,401