Investments - Available-for-sale securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale
Amortized Cost	$ 388,768	$ 1,355,614
Gross Unrealized Gains		118
Gross Unrealized Losses		(22,789)
Fair Value	388,768	1,332,943
Corporate bonds
Debt Securities, Available-for-sale
Amortized Cost	128,111	200,735
Gross Unrealized Gains		7
Gross Unrealized Losses		(7,109)
Fair Value	128,111	193,633
U.S. Treasuries
Debt Securities, Available-for-sale
Amortized Cost	260,657	1,154,879
Gross Unrealized Gains		111
Gross Unrealized Losses		(15,680)
Fair Value	$ 260,657	$ 1,139,310