Stock-Based Compensation - Summary of Time Based and Performance-Based Stock Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	$ 13,657	$ 11,580	$ 41,580	$ 45,684
Research and development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	5,403	4,279	17,474	23,322
General and administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	$ 8,254	$ 7,301	$ 24,106	$ 22,362