Net Loss per Share (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net Loss per Share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	11,705	12,645
Outstanding stock options
Net Loss per Share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	8,067	7,366
Unvested restricted stock units
Net Loss per Share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	2,197	1,093
Shares issuable upon conversion of Convertible Notes
Net Loss per Share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1,441	4,186