Investments - Amortized Cost (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Cost	$ 515,799	$ 409,996
Gross Unrealized Gains	113	47
Gross Unrealized Losses	(106)	(12)
Fair Value	515,806	410,031
Included in cash and cash equivalents
Debt Securities, Available-for-sale [Line Items]
Cost	86,520	133,319
Gross Unrealized Gains	4	4
Gross Unrealized Losses	(1)	(2)
Fair Value	86,523	133,321
Included in marketable securities
Debt Securities, Available-for-sale [Line Items]
Cost	429,279	276,677
Gross Unrealized Gains	109	43
Gross Unrealized Losses	(105)	(10)
Fair Value	429,283	276,710
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Cost	185,504	155,339
Gross Unrealized Gains	12	14
Gross Unrealized Losses	(81)	(6)
Fair Value	185,435	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	(3)	0
Fair Value	4,991	19,996
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Cost	226,610	182,218
Gross Unrealized Gains	18	6
Gross Unrealized Losses	(8)	(4)
Fair Value	226,620	182,220
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Cost	84,672	45,431
Gross Unrealized Gains	71	21
Gross Unrealized Losses	(14)	(2)
Fair Value	84,729	45,450
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost	14,019	7,012
Gross Unrealized Gains	12	6
Gross Unrealized Losses	0	0
Fair Value	$ 14,031	$ 7,018