Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,146,055	$ 6,178,123
Gross Unrealized Gains	29,999	19,722
Gross Unrealized Losses	(291,021)	(346,156)
Fair Value	5,885,033	5,851,689
Assets held by insurance regulatory requirements	12,300	12,200
Assets held in trust for the benefit of contractual counterparties	182,800	199,900
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	275,166	265,349
Gross Unrealized Gains	340	231
Gross Unrealized Losses	(3,812)	(5,087)
Fair Value	271,694	260,493
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,026,899	2,065,953
Gross Unrealized Gains	3,915	2,331
Gross Unrealized Losses	(165,119)	(192,789)
Fair Value	1,865,695	1,875,495
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,858,150	2,857,627
Gross Unrealized Gains	18,082	12,593
Gross Unrealized Losses	(93,465)	(112,839)
Fair Value	2,782,767	2,757,381
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	180,604	155,594
Gross Unrealized Gains	2,702	2,157
Gross Unrealized Losses	(915)	(1,234)
Fair Value	182,391	156,517
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	375,046	373,485
Gross Unrealized Gains	4,463	2,103
Gross Unrealized Losses	(21,520)	(25,528)
Fair Value	357,989	350,060
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	264,095	243,840
Gross Unrealized Gains	421	21
Gross Unrealized Losses	(5,571)	(7,990)
Fair Value	258,945	235,871
CLOs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	146,537	199,773
Gross Unrealized Gains	76	286
Gross Unrealized Losses	(32)	0
Fair Value	146,581	200,059
Foreign government debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,487	4,487
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(587)	(689)
Fair Value	3,900	3,798
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,071	12,015
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 15,071	$ 12,015