Basis of Presentation and Significant Accounting Policies - Computation of Net Loss per Common Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
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/units at end of period	20,862	19,181	20,862	19,181
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units	2,116	929	2,743	1,112
Common stock equivalents under if-converted method for convertible notes (in shares)		501		501