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, 2024	Dec. 31, 2023	Sep. 30, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate Fair Value	$ 168,350	$ 241,576	$ 452,100
Gross Unrealized Holding Gains	3,035	1,044
Gross Unrealized Holding Losses	(27)	(130)
Equity securities	5,500
Amortized Cost	1,177,917	1,621,500
Aggregate Fair Value	1,186,442	1,632,267
Level 1 | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	118,278	278,187
Aggregate Fair Value	118,278	278,187
Restricted cash equivalents	21,000	19,700
Level 1 | Equity securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	5,517	9,853
Level 2 | U.S. government treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	691,089	1,162,124
Gross Unrealized Holding Gains	1,469	1,017
Gross Unrealized Holding Losses	(25)	(80)
Aggregate Fair Value	692,533	1,163,061
Level 2 | U.S. government agency bonds and discount notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	53,529	181,189
Gross Unrealized Holding Gains	53	27
Gross Unrealized Holding Losses	(1)	(50)
Aggregate Fair Value	53,581	$ 181,166
Level 2 | Asset-backed securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	37,083
Gross Unrealized Holding Gains	278
Gross Unrealized Holding Losses	0
Aggregate Fair Value	37,361
Level 2 | Corporate bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	277,938
Gross Unrealized Holding Gains	1,235
Gross Unrealized Holding Losses	(1)
Aggregate Fair Value	$ 279,172