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Stock-Based Compensation (Performance Stock Units) (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Jul. 31, 2018
Mar. 31, 2018
Mar. 31, 2017
Nov. 30, 2016
May 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Unvested activity                
Nonvested at the beginning of the period (in shares)           743,583    
Granted (in shares)           240,008    
Forfeited (in shares)           (34,426)    
Cancellations (in shares)           (454,283)    
Nonvested at the end of the period (in shares)           494,882 743,583  
Performance Stock Units                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Performance period           3 years    
Fair value assumptions                
Risk-free interest rate (as a percent) 2.69% 2.36% 1.54% 0.96% 0.84%      
Dividend yield (as a percent) 0.00% 0.00% 0.00% 0.00% 0.00%      
Expected Life (Years) 2 years 5 months 1 day 2 years 9 months 29 days 2 years 9 months 29 days 2 years 1 month 20 days 2 years 7 months 24 days      
Volatility 33.10% 34.70% 34.20% 35.20% 33.30%      
Fair value of award (in dollars per share) $ 39.06 $ 29.56 $ 17.05 $ 17.84 $ 7.19 $ 29.83    
Recognized stock-based compensation expense           $ 4.8 $ 3.5 $ 2.5
Unrecognized stock-based compensation expense           $ 5.9    
Unrecognized stock-based compensation expense, period of recognition           1 year 9 months 18 days    
Unvested activity                
Granted (in shares) 6,742 233,266 286,251 9,524 527,249      
Weighted Average Grant Date Fair Value                
Nonvested at the beginning of the period (in dollars per share)           $ 10.91    
Granted (in dollars per share) $ 39.06 $ 29.56 $ 17.05 $ 17.84 $ 7.19 29.83    
Forfeited (in dollars per share)           13.33    
Cancellations (in dollars per share)           7.41    
Nonvested at the end of the period (in dollars per share)           $ 23.13 $ 10.91  
Shares expected to be issued (in shares)           908,566    
Performance Stock Units | Minimum                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Payout of PSUs (percent of initial grant)           0.00%    
Performance Stock Units | Maximum                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Payout of PSUs (percent of initial grant)           200.00%