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Stock-Based Compensation - Valuation Assumptions (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Options granted (shares) 0 0 172 750
Options granted, weighted average exercise price (usd per share) $ 0.00 $ 0.00 $ 24.77 $ 12.85
Weighted average grant-date fair value per share (usd per share) $ 0.00 $ 0.00 $ 9.58 $ 4.85
Stock Options and PSOs        
Weighted average Black-Scholes model assumptions:        
Risk-free interest rate 0.00% 0.00% 2.02% 1.45%
Expected term (in years) 0 years 0 years 6 years 4 months 24 days 5 years 10 months 24 days
Expected volatility 0.00% 0.00% 35.00% 37.00%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%
Remaining stock-based compensation expense for unvested options, not yet recognized $ 2.7   $ 2.7  
Stock-based compensation expense, weighted average recognition period     1 year 5 months 18 days