Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,027,249	$ 6,178,123
Gross Unrealized Gains	41,822	19,722
Gross Unrealized Losses	(266,740)	(346,156)
Fair Value	5,802,331	5,851,689
Assets held by insurance regulatory requirements	12,900	12,200
Assets held in trust for the benefit of contractual counterparties	203,600	199,900
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	341,550	265,349
Gross Unrealized Gains	596	231
Gross Unrealized Losses	(3,452)	(5,087)
Fair Value	338,694	260,493
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,994,628	2,065,953
Gross Unrealized Gains	4,391	2,331
Gross Unrealized Losses	(161,614)	(192,789)
Fair Value	1,837,405	1,875,495
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,726,805	2,857,627
Gross Unrealized Gains	27,734	12,593
Gross Unrealized Losses	(76,098)	(112,839)
Fair Value	2,678,441	2,757,381
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	183,698	155,594
Gross Unrealized Gains	3,153	2,157
Gross Unrealized Losses	(696)	(1,234)
Fair Value	186,155	156,517
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	379,179	373,485
Gross Unrealized Gains	4,742	2,103
Gross Unrealized Losses	(20,642)	(25,528)
Fair Value	363,279	350,060
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	248,872	243,840
Gross Unrealized Gains	963	21
Gross Unrealized Losses	(3,735)	(7,990)
Fair Value	246,100	235,871
CLOs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	124,435	199,773
Gross Unrealized Gains	243	286
Gross Unrealized Losses	0	0
Fair Value	124,678	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	(503)	(689)
Fair Value	3,984	3,798
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,595	12,015
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 23,595	$ 12,015