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Stock-Based Compensation - Fair Value Assumptions (Details) - Employee stock options
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Stock-based awards to employees - weighted average assumptions used to estimate fair value of options granted        
Risk-free interest rate (as a percent) 2.01% 1.45% 2.13% 1.58%
Expected dividend yield (as a percent) 0.00% 0.00% 0.00% 0.00%
Expected volatility rate (as a percent) 46.60% 53.20% 46.60% 52.80%
Expected term 6 years 1 month 2 days 5 years 9 months 11 days 6 years 1 month 2 days 6 years 18 days