Marketable Securities - Summary of Cash Equivalents and Marketable Securities Recorded at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Cash equivalents, amortized cost	$ 631,890	$ 773,560
Cash equivalents, gross unrealized gains	1	1
Cash equivalents, gross unrealized losses	(2)	(24)
Cash equivalents, fair value	631,889	773,537
Marketable securities, amortized cost	680,657	521,820
Marketable securities, gross unrealized gains	39	51
Marketable securities, gross unrealized losses	(552)	(158)
Marketable securities, fair value	680,144	521,713
Cash equivalents and marketable securities, amortized cost	1,312,547	1,295,380
Cash equivalents and marketable securities, gross unrealized gains	40	52
Cash equivalents and marketable securities, gross unrealized losses	(554)	(182)
Cash equivalents and marketable securities, fair value	1,312,033	1,295,250
Money Market Funds [Member]
Marketable Securities [Line Items]
Cash equivalents, amortized cost	629,357	742,958
Cash equivalents, fair value	629,357	742,958
Certificates of Deposit [Member]
Marketable Securities [Line Items]
Cash equivalents, amortized cost		12,527
Cash equivalents, fair value		12,527
Marketable securities, amortized cost	54,273	25,162
Marketable securities, fair value	54,273	25,162
Commercial Paper [Member]
Marketable Securities [Line Items]
Cash equivalents, amortized cost		15,549
Cash equivalents, fair value		15,549
Marketable securities, amortized cost	87,120	90,375
Marketable securities, gross unrealized gains	2
Marketable securities, fair value	87,122	90,375
US Treasury Securities [Member]
Marketable Securities [Line Items]
Marketable securities, amortized cost		47,976
Marketable securities, gross unrealized gains		3
Marketable securities, fair value		47,979
Corporate Debt Securities [Member]
Marketable Securities [Line Items]
Cash equivalents, amortized cost	2,533	2,526
Cash equivalents, gross unrealized gains	1	1
Cash equivalents, gross unrealized losses	(2)	(24)
Cash equivalents, fair value	2,532	2,503
Marketable securities, amortized cost	536,144	324,569
Marketable securities, gross unrealized gains	37	43
Marketable securities, gross unrealized losses	(551)	(156)
Marketable securities, fair value	535,630	324,456
Government-sponsored Enterprise Securities [Member]
Marketable Securities [Line Items]
Marketable securities, amortized cost	3,120	33,738
Marketable securities, gross unrealized gains		5
Marketable securities, gross unrealized losses	(1)	(2)
Marketable securities, fair value	$ 3,119	$ 33,741