Summary of Significant Accounting Policies - Computation of Net Loss per Common Share (Details) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 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	14,815	17,380	14,290
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock	1,020	3,001	1,579
Shares issuable upon conversion of convertible notes at end of period (in shares) | $	501	501	501
Common stock equivalents under if-converted method for convertible notes (in shares)	501	501	501