Net Loss per Share (Details) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Net Loss per Share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	12,294	7,667
Outstanding stock options
Net Loss per Share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	7,204	7,080
Unvested restricted stock units
Net Loss per Share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1,158	587
Shares issuable upon conversion of Convertible Notes
Net Loss per Share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	3,932