Investments - Amortized Cost (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Cost	$ 417,891	$ 409,996
Gross Unrealized Gains	174	47
Gross Unrealized Losses	(75)	(12)
Fair Value	417,990	410,031
Included in cash and cash equivalents
Debt Securities, Available-for-sale [Line Items]
Cost	47,193	133,319
Gross Unrealized Gains	1	4
Gross Unrealized Losses	0	(2)
Fair Value	47,194	133,321
Included in marketable securities
Debt Securities, Available-for-sale [Line Items]
Cost	370,698	276,677
Gross Unrealized Gains	173	43
Gross Unrealized Losses	(75)	(10)
Fair Value	370,796	276,710
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Cost	120,260	155,339
Gross Unrealized Gains	13	14
Gross Unrealized Losses	(63)	(6)
Fair Value	120,210	155,347
U.S. agency and government sponsored securities
Debt Securities, Available-for-sale [Line Items]
Cost	4,994	19,996
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4,994	19,996
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Cost	132,991	182,218
Gross Unrealized Gains	5	6
Gross Unrealized Losses	(2)	(4)
Fair Value	132,994	182,220
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Cost	138,832	45,431
Gross Unrealized Gains	147	21
Gross Unrealized Losses	(9)	(2)
Fair Value	138,970	45,450
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost	20,814	7,012
Gross Unrealized Gains	9	6
Gross Unrealized Losses	(1)	0
Fair Value	$ 20,822	$ 7,018