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Equity-Based Compensation Plans Summary of Status of Outstanding RSU Awards (Details) - Restricted stock units - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value $ 1,100    
Number of share      
Beginning Balance 54,460 46,029 93,254
Granted 66,774 44,627 23,148
Vested (17,531) (29,338) (65,217)
Forfeited (9,232) (6,858) (5,156)
Ending Balance 94,471 54,460 46,029
Weighted Average Grant Date Fair Value      
Beginning Balance $ 47.16 $ 60.17 $ 47.95
Granted 56.45 39.10 62.02
Vested 62.75 55.16 43.86
Forfeited 53.52 47.80 58.58
Ending Balance $ 50.22 $ 47.16 $ 60.17
Aggregate intrinsic value      
Aggregate intrinsic value $ 4,289 $ 3,570 $ 2,096
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized $ 2,800    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 9 months