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Stock-Based Compensation (Performance Stock Units) (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Mar. 31, 2017
Nov. 30, 2016
May 31, 2016
May 31, 2015
Nov. 30, 2014
Sep. 30, 2014
Aug. 31, 2014
Jul. 31, 2014
May 31, 2014
Apr. 30, 2014
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Unvested activity                          
Nonvested at the beginning of the period (in shares)                     692,562    
Granted (in shares)                     286,251    
Forfeited (in shares)                     (68,387)    
Cancellations (in shares)                     (166,843)    
Nonvested at the end of the period (in shares)                     743,583 692,562  
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) 1.54% 0.96% 0.84% 0.83% 0.55% 0.72% 0.67% 0.74%   0.75%      
Dividend yield (as a percent) 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%   0.00%      
Expected Life (Years) 2 years 303 days 2 years 51 days 2 years 237 days 2 years 237 days 2 years 1 month 6 days 2 years 3 months 15 days 2 years 5 months 3 days 2 years 5 months 3 days   2 years 8 months 24 days      
Volatility 34.20% 35.20% 33.30% 28.50% 26.30% 25.70% 26.90% 26.90%   34.30%      
Fair value of award (in dollars per share) $ 17.05 $ 17.84 $ 7.19 $ 14.48 $ 7.42 $ 15.26 $ 21.86 $ 22.31   $ 18.00 $ 17.05    
Recognized stock-based compensation expense                     $ 3.5 $ 2.5 $ 1.4
Unrecognized stock-based compensation expense                     $ 3.9    
Unrecognized stock-based compensation expense, period of recognition                     1 year 256 days    
Unvested activity                          
Granted (in shares) 286,251 9,524 527,249 237,063                  
Weighted Average Grant Date Fair Value                          
Nonvested at the beginning of the period (in dollars per share)                     $ 9.25    
Granted (in dollars per share) $ 17.05 $ 17.84 $ 7.19 $ 14.48 $ 7.42 $ 15.26 $ 21.86 $ 22.31   $ 18.00 17.05    
Forfeited (in dollars per share)                     11.04    
Cancellations (in dollars per share)                     14.48    
Nonvested at the end of the period (in dollars per share)                     $ 10.91 $ 9.25  
Performance Stock Units | May 2014 Grant 1                          
Fair value assumptions                          
Risk-free interest rate (as a percent)                 0.70%        
Dividend yield (as a percent)                 0.00%        
Expected Life (Years)                 2 years 7 months 26 days        
Volatility                 31.80%        
Fair value of award (in dollars per share)                 $ 25.26        
Weighted Average Grant Date Fair Value                          
Granted (in dollars per share)                 $ 25.26        
Performance Stock Units | May 2014 Grant 2                          
Fair value assumptions                          
Risk-free interest rate (as a percent)                 0.63%        
Dividend yield (as a percent)                 0.00%        
Expected Life (Years)                 2 years 7 months 11 days        
Volatility                 30.10%        
Fair value of award (in dollars per share)                 $ 27.53        
Weighted Average Grant Date Fair Value                          
Granted (in dollars per share)                 $ 27.53        
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%