Investments - Schedule of Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,586,730	$ 5,184,856
Unrealized Gains	13,446	4,273
Unrealized Losses	(336,437)	(447,504)
Fair Value	5,263,739	4,741,625
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,014,076	584,173
Unrealized Gains	1,434	341
Unrealized Losses	(19,128)	(28,076)
Fair Value	996,382	556,438
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,199	49,059
Unrealized Gains	0	7
Unrealized Losses	(4)	(8)
Fair Value	7,195	49,058
U.S. agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	922,907	898,675
Unrealized Gains	438	258
Unrealized Losses	(101,999)	(115,190)
Fair Value	821,346	783,743
Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	585,047	661,934
Unrealized Gains	6,660	2,010
Unrealized Losses	(44,449)	(61,254)
Fair Value	547,258	602,690
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	77,516	69,651
Unrealized Gains	0	0
Unrealized Losses	(10,069)	(7,252)
Fair Value	67,447	62,399
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,380,533	1,546,513
Unrealized Gains	4,425	1,195
Unrealized Losses	(87,903)	(133,387)
Fair Value	1,297,055	1,414,321
Residential and commercial mortgage securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	571,163	577,915
Unrealized Gains	286	390
Unrealized Losses	(53,509)	(66,481)
Fair Value	517,940	511,824
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	584,168	660,345
Unrealized Gains	203	72
Unrealized Losses	(19,376)	(35,856)
Fair Value	564,995	624,561
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	444,121	136,591
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	$ 444,121	$ 136,591

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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