Fair Value Measurements - Financial Assets Measured at Fair Value on a Recurring Basis by Level Within Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2020		Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets, Amortized Cost	$ 836,112		$ 406,883
Financial Assets, Gross Unrealized Holding Gains	235		140
Financial Assets, Gross Unrealized Holding Losses	(1)		(5)
Financial Assets, Aggregate Fair Value	836,346		407,018
Level 1 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets, Amortized Cost	472,272	[1]	106,127	[2]
Financial Assets, Aggregate Fair Value	472,272	[1]	106,127	[2]
Level 2 | U.S. Government Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets, Amortized Cost	363,840		298,256	[3]
Financial Assets, Gross Unrealized Holding Gains	235		140	[3]
Financial Assets, Gross Unrealized Holding Losses	(1)		(5)	[3]
Financial Assets, Aggregate Fair Value	$ 364,074		298,391	[3]
Level 2 | Bank Time Deposits
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets, Amortized Cost			2,500
Financial Assets, Aggregate Fair Value			$ 2,500

[1] Includes $10.6 million of restricted cash equivalents.

[2] Includes $13.5 million of restricted cash equivalents.

[3] Includes $24.3 million classified as long-term investments.