Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Assets held by insurance regulatory requirements	$ 12,100	$ 12,200
Assets held in trust for the benefit of contractual counterparties	180,200	156,900
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,023,778	6,061,022
Gross Unrealized Gains	18,948	28,235
Gross Unrealized Losses	(370,012)	(366,144)
Fair Value	5,672,714	5,723,113
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	147,441	167,995
Gross Unrealized Gains	66	51
Gross Unrealized Losses	(6,595)	(6,364)
Fair Value	140,912	161,682
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,079,844	2,092,754
Gross Unrealized Gains	3,042	5,159
Gross Unrealized Losses	(191,274)	(189,835)
Fair Value	1,891,612	1,908,078
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,644,912	2,626,401
Gross Unrealized Gains	10,789	17,391
Gross Unrealized Losses	(129,340)	(128,211)
Fair Value	2,526,361	2,515,581
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	173,761	173,256
Gross Unrealized Gains	1,291	1,292
Gross Unrealized Losses	(2,536)	(3,275)
Fair Value	172,516	171,273
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	345,029	347,132
Gross Unrealized Gains	3,338	4,297
Gross Unrealized Losses	(24,541)	(20,656)
Fair Value	323,826	330,773
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	286,876	293,204
Gross Unrealized Gains	5	5
Gross Unrealized Losses	(14,503)	(15,752)
Fair Value	272,378	277,457
CLOs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	318,683	327,467
Gross Unrealized Gains	415	37
Gross Unrealized Losses	(482)	(1,408)
Fair Value	318,616	326,096
Foreign government debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,487	4,486
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(741)	(643)
Fair Value	3,746	3,843
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,745	28,327
Gross Unrealized Gains	2	3
Gross Unrealized Losses	0	0
Fair Value	$ 22,747	$ 28,330