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Stock-based Compensation - Performance-Based RSUs (Details)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Apr. 30, 2019
shares
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
Performance-based RSUs        
Summary of equity award grants, RSUs        
Balance at beginning of period (in units)   2,389,000 2,475,000 2,350,000
Granted (in units)   524,000 694,000 813,000
Vested (in units) (649,000) (730,000) (446,000)  
Forfeited or canceled (in units)   (291,000) (334,000) (688,000)
Balance at end of period (in units)   1,892,000 2,389,000 2,475,000
Compensation expense recorded during the year | $   $ 12,425 $ 11,020 $ 9,475
Weighted-average grant date fair value per unit:        
Outstanding at beginning of year (in dollars per share) | $ / shares   $ 15.57 $ 14.24 $ 16.30
Granted (in dollars per share) | $ / shares   24.40 18.04 11.28
Vested (in dollars per share) | $ / shares   12.86 13.65  
Forfeited (in dollars per share) | $ / shares   16.17 14.45 16.87
Outstanding at end of year (in dollars per share) | $ / shares   $ 14.48 $ 15.57 $ 14.24
Unamortized compensation cost | $   $ 5,805    
Number of shares expected to vest (in units)   1,784,000    
Weighted average remaining vesting period (in months)   11 months    
Actually vested (in shares)   774,000    
Vesting percentage   106    
Minimum | Performance-based RSUs        
Stock-Based Compensation        
Shares earned as a percent of performance goal achievement   0.00%    
Fair Value Assumptions        
Expected grantee forfeiture rate (as a percent)   0.00%    
Maximum | Performance-based RSUs        
Stock-Based Compensation        
Shares earned as a percent of performance goal achievement   130.00%    
Fair Value Assumptions        
Expected grantee forfeiture rate (as a percent)   23.20%