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Stock-Based Compensation - Weighted-Average Assumptions (Details) - PRSUs
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (in years) 2 years 11 months 1 day 2 years 10 months 10 days
Expected volatility 42.10% 42.70%
Risk-free interest rate 1.51% 0.88%
Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Correlation range 26.80% 24.40%
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Correlation range 76.00% 71.00%