Annual report pursuant to Section 13 and 15(d)

Stock-Based Payments - Weighted-Average Assumptions to Estimate Fair Value of Stock Options (Details)

v3.8.0.1
Stock-Based Payments - Weighted-Average Assumptions to Estimate Fair Value of Stock Options (Details) - Stock options - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Weighted-average assumptions used in Black-Scholes option-pricing model to estimate fair value      
Weighted-average risk-free interest rate (as a percent) 2.07% 1.36% 1.78%
Expected option term 6 years 18 days 5 years 11 months 23 days 6 years 1 month 2 days
Volatility (as a percent) 87.83% 85.39% 82.71%
Weighted-average grant date fair value of options granted (in dollars per share) $ 8.75 $ 8.58 $ 4.88