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Stock-based Compensation - Time-Based RSUs (Details)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Summary of equity award grants, RSUs      
Compensation expense recorded during the year | $ $ 25,251 $ 20,697 $ 16,505
Time-based RSUs      
Stock-Based Compensation      
Percentage of the award vesting at each of the three anniversaries 33.00%    
Summary of equity award grants, RSUs      
Balance at beginning of period (in units) 600,000 382,000 271,000
Granted (in units) 328,000 408,000 261,000
Vested (in units) (254,000) (173,000) (127,000)
Forfeited or canceled (in units) (47,000) (17,000) (23,000)
Balance at end of period (in units) 627,000 600,000 382,000
Compensation expense recorded during the year | $ $ 6,608 $ 4,768 $ 2,494
Weighted-average grant date fair value per unit:      
Outstanding at beginning of year (in dollars per share) | $ / shares $ 16.37 $ 13.71 $ 17.81
Granted (in dollars per share) | $ / shares 24.25 18.02 11.77
Vested (in dollars per share) | $ / shares 16.08 14.66 17.99
Forfeited (in dollars per share) | $ / shares 19.40 14.87 15.55
Outstanding at end of year (in dollars per share) | $ / shares $ 20.39 $ 16.37 $ 13.71
Unamortized compensation cost | $ $ 4,158    
Number of shares expected to vest (in units) 579,000    
Weighted average remaining vesting period (in months) 11 months    
Time-based RSUs | Minimum      
Stock-Based Compensation      
Turnover rates (as a percent) 0.0    
Time-based RSUs | Maximum      
Stock-Based Compensation      
Turnover rates (as a percent) 23.2    
Time-based RSUs | Common Stock      
Stock-Based Compensation      
Number of share awarded for each RSU (in shares) 1