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Stock-Based Compensation - Summary of stock option activity (Details) - Time Based [Member] - $ / shares
shares in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number 191 191 213 230
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price $ 21.37 $ 21.37 $ 21.50 $ 20.73
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (21) (37)    
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price $ 22.90 $ 18.03    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Expirations in Period (1) (2)    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price $ 11.21 $ 8.95