Organization, Basis of Presentation and Liquidity (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($) program segment divestiture	Dec. 31, 2020 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of late-stage development programs | program	2
Number of operating segments | segment	1
Total cash, cash equivalents and short-term investments, estimated fair value	$ 343,024	$ 505,109
Number of discrete business divestitures | divestiture	2
Accumulated deficit	$ 1,497,370	$ 1,324,840