Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,036,944	$ 6,178,123
Gross Unrealized Gains	60,929	19,722
Gross Unrealized Losses	(229,279)	(346,156)
Fair Value	5,868,594	5,851,689
Assets held by insurance regulatory requirements	12,600	12,200
Assets held in trust for the benefit of contractual counterparties	190,900	199,900
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	339,344	265,349
Gross Unrealized Gains	968	231
Gross Unrealized Losses	(2,813)	(5,087)
Fair Value	337,499	260,493
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,002,827	2,065,953
Gross Unrealized Gains	9,876	2,331
Gross Unrealized Losses	(140,274)	(192,789)
Fair Value	1,872,429	1,875,495
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,730,841	2,857,627
Gross Unrealized Gains	36,854	12,593
Gross Unrealized Losses	(64,153)	(112,839)
Fair Value	2,703,542	2,757,381
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	206,520	155,594
Gross Unrealized Gains	3,500	2,157
Gross Unrealized Losses	(1,354)	(1,234)
Fair Value	208,666	156,517
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	372,361	373,485
Gross Unrealized Gains	7,289	2,103
Gross Unrealized Losses	(17,804)	(25,528)
Fair Value	361,846	350,060
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	224,852	243,840
Gross Unrealized Gains	2,231	21
Gross Unrealized Losses	(2,461)	(7,990)
Fair Value	224,622	235,871
CLOs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	117,798	199,773
Gross Unrealized Gains	211	286
Gross Unrealized Losses	(23)	0
Fair Value	117,986	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	(397)	(689)
Fair Value	4,090	3,798
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,914	12,015
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 37,914	$ 12,015