Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 6,784
Marketable securities, fair value	680,144	$ 521,713
Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other non-current assets	2,212	600
Total	1,808,415	1,690,933
Recurring Basis [Member] | Cash [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	494,170	395,083
Recurring Basis [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	629,357	742,958
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	2,532	2,503
Marketable securities, fair value	535,630	324,456
Recurring Basis [Member] | Certificates of Deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents		12,527
Marketable securities, fair value	54,273	25,162
Recurring Basis [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents		15,549
Marketable securities, fair value	87,122	90,375
Recurring Basis [Member] | US Treasury Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, fair value		47,979
Recurring Basis [Member] | Government-sponsored Enterprise Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, fair value	3,119	33,741
Recurring Basis [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	1,123,527	1,138,041
Recurring Basis [Member] | Level 1 [Member] | Cash [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	494,170	395,083
Recurring Basis [Member] | Level 1 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	629,357	742,958
Recurring Basis [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	682,676	552,292
Recurring Basis [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	2,532	2,503
Marketable securities, fair value	535,630	324,456
Recurring Basis [Member] | Level 2 [Member] | Certificates of Deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents		12,527
Marketable securities, fair value	54,273	25,162
Recurring Basis [Member] | Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents		15,549
Marketable securities, fair value	87,122	90,375
Recurring Basis [Member] | Level 2 [Member] | US Treasury Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, fair value		47,979
Recurring Basis [Member] | Level 2 [Member] | Government-sponsored Enterprise Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, fair value	3,119	33,741
Recurring Basis [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other non-current assets	2,212	600
Total	$ 2,212	$ 600