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Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Stock-Based Compensation      
Total stock-based compensation $ 8.0 $ 9.4 $ 6.6
Estimated forfeiture rate (as a percent) 5.80% 5.10% 7.00%
Stock options      
Stock-Based Compensation      
Total stock-based compensation $ 7.1 $ 6.7 $ 5.3
Risk-free interest rate, minimum (as a percent) 1.58% 1.78% 1.07%
Risk-free interest rate, maximum (as a percent) 1.91% 2.10% 2.45%
Expected life     6 years 6 months
Volatility (as a percent)     54.90%
Volatility, minimum (as a percent) 35.10% 53.07%  
Volatility, maximum (as a percent) 52.23% 56.24%  
Stock options | Minimum      
Stock-Based Compensation      
Vesting period 3 years    
Expected life 6 years 6 years  
Stock options | Maximum      
Stock-Based Compensation      
Vesting period 5 years    
Expected life 6 years 3 months 6 years 3 months  
Restricted stock      
Stock-Based Compensation      
Total stock-based compensation $ 0.9 $ 2.7 $ 1.3