Summary of Significant Accounting Policies - Computation of Net Loss per Common Share (Details) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($) 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	18,754	18,153	18,754	18,153
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock	731	3,153	926	3,378
Shares issuable upon conversion of convertible notes at end of period (in shares) | $	501	501	501	501
Common stock equivalents under if-converted method for convertible notes (in shares)	501	501	501	501