Investments - Amortized Cost (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Cost	$ 552,905	$ 409,996
Gross Unrealized Gains	77	47
Gross Unrealized Losses	(86)	(12)
Fair Value	552,896	410,031
Included in cash and cash equivalents
Debt Securities, Available-for-sale [Line Items]
Cost	166,249	133,319
Gross Unrealized Gains	6	4
Gross Unrealized Losses	0	(2)
Fair Value	166,255	133,321
Included in marketable securities
Debt Securities, Available-for-sale [Line Items]
Cost	386,656	276,677
Gross Unrealized Gains	71	43
Gross Unrealized Losses	(86)	(10)
Fair Value	386,641	276,710
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Cost	210,739	155,339
Gross Unrealized Gains	29	14
Gross Unrealized Losses	(59)	(6)
Fair Value	210,709	155,347
U.S. agency and government sponsored securities
Debt Securities, Available-for-sale [Line Items]
Cost		19,996
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		19,996
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Cost	272,457	182,218
Gross Unrealized Gains	15	6
Gross Unrealized Losses	(8)	(4)
Fair Value	272,464	182,220
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Cost	61,662	45,431
Gross Unrealized Gains	29	21
Gross Unrealized Losses	(19)	(2)
Fair Value	61,672	45,450
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost	8,047	7,012
Gross Unrealized Gains	4	6
Gross Unrealized Losses	0	0
Fair Value	$ 8,051	$ 7,018