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Stock-Based Compensation Vested RSUs (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period 4 years    
Common stock, shares issued 51,104,593 50,415,152  
Restricted Stock Units (RSUs) | Common stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated Share-based Compensation Expense $ 40,868    
Stock Incentive Plan | Common stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Shares Authorized 20,000,000.0    
Stock Incentive Plan | Restricted Stock Units (RSUs) | Common stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock, shares issued 15,800,000    
Allocated Share-based Compensation Expense $ 4,216 $ 3,957 $ 4,685
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested, Number of Shares [Roll Forward] [Roll Forward]      
Balance at Beginning of Period (shares) 42,000 34,000 30,000
Granted (shares) 15,000 16,000 13,000
Delivered (shares) (12,000) 8,000 9,000
Balance at End of Period (shares) 45,000 42,000 34,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested, Weighted Average Grant Date Fair Value [Abstract] [Abstract]      
Balance at beginning of period, Weighted Average Grant Date Fair Value (usd per share) $ 36.98 $ 37.15 $ 36.17
Granted, Weighted Average Grant Date Fair Value (usd per share) 38.29 38.14 37.17
Delivered, Weighted Average Grant Date Fair Value (usd per share) 34.95 40.03 34.02
Balance at end of period, Weighted Average Grant Date Fair Value (usd per share) $ 37.93 $ 36.98 $ 37.15
Stock Incentive Plan | Vested Restricted Stock Units (RSUs) | Common stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated Share-based Compensation Expense $ 626 $ 618 $ 486