Summary of Significant Accounting Policies - Computation of Net Loss per Common Share (Details) shares in Thousands, Unit14 in Thousands	3 Months Ended
	Mar. 31, 2019 shares	Mar. 31, 2018 shares
Computation of Net Loss per Common Share
Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock at end of period	21,528	17,677
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock	1,559	3,436
Shares issuable upon conversion of convertible notes at end of period (in shares)	501	501
Common stock equivalents under if-converted method for convertible notes (in shares)	501	501