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Stock-Based Compensation - Fair Value of Employee Option Grant Estimated on Date of Grant (Details) (USD $)
3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value $ 10.18 $ 6.77 $ 8.39 $ 6.73
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Expected volatility 70.00% 88.00% 82.00% 87.00%
Expected term, in years 6 years 0 months 25 days 5 years 9 months 17 days 5 years 11 months 8 days 5 years 9 months 17 days
Risk-free interest rate 1.95% 1.66% 1.90% 1.65%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%