Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,150,471
Fair Value	5,789,968
Assets held by insurance regulatory requirements	12,100	$ 12,200
Assets held in trust for the benefit of contractual counterparties	197,900	156,900
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,150,471	6,061,022
Gross Unrealized Gains	14,226	28,235
Gross Unrealized Losses	(374,729)	(366,144)
Fair Value	5,789,968	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	270,158	167,995
Gross Unrealized Gains	66	51
Gross Unrealized Losses	(6,311)	(6,364)
Fair Value	263,913	161,682
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,068,272	2,092,754
Gross Unrealized Gains	2,141	5,159
Gross Unrealized Losses	(197,033)	(189,835)
Fair Value	1,873,380	1,908,078
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,654,262	2,626,401
Gross Unrealized Gains	7,893	17,391
Gross Unrealized Losses	(130,214)	(128,211)
Fair Value	2,531,941	2,515,581
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	174,794	173,256
Gross Unrealized Gains	1,098	1,292
Gross Unrealized Losses	(2,020)	(3,275)
Fair Value	173,872	171,273
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	381,258	347,132
Gross Unrealized Gains	2,459	4,297
Gross Unrealized Losses	(25,400)	(20,656)
Fair Value	358,317	330,773
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	275,559	293,204
Gross Unrealized Gains	26	5
Gross Unrealized Losses	(12,759)	(15,752)
Fair Value	262,826	277,457
CLOs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	291,461	327,467
Gross Unrealized Gains	542	37
Gross Unrealized Losses	(275)	(1,408)
Fair Value	291,728	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	(717)	(643)
Fair Value	3,770	3,843
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	30,220	28,327
Gross Unrealized Gains	1	3
Gross Unrealized Losses	0	0
Fair Value	$ 30,221	$ 28,330