Stock-Based Compensation - Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Compensation expense:
Total stock-based compensation expense	$ 65,705	$ 2,624	$ 77,602	$ 12,803
Additional disclosures:
Unrecognized compensation expense	311,000		$ 311,000
Unrecognized compensation expense, recognition period			1 year 4 months 24 days
Research and development
Compensation expense:
Total stock-based compensation expense	28,730	723	$ 34,735	6,347
General and administrative
Compensation expense:
Total stock-based compensation expense	$ 36,975	$ 1,901	$ 42,867	$ 6,456