Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fixed income, available-for-sale	$ 5,851,689	$ 5,723,113
Assets held by insurance regulatory requirements	12,200	12,200
Assets held in trust for the benefit of contractual counterparties	199,900	156,900
Fixed income securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	6,178,123	6,061,022
Gross Unrealized Gains	19,722	28,235
Gross Unrealized Losses	(346,156)	(366,144)
Fixed income, available-for-sale	5,851,689	5,723,113
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	265,349	167,995
Gross Unrealized Gains	231	51
Gross Unrealized Losses	(5,087)	(6,364)
Fixed income, available-for-sale	260,493	161,682
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,065,953	2,092,754
Gross Unrealized Gains	2,331	5,159
Gross Unrealized Losses	(192,789)	(189,835)
Fixed income, available-for-sale	1,875,495	1,908,078
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,857,627	2,626,401
Gross Unrealized Gains	12,593	17,391
Gross Unrealized Losses	(112,839)	(128,211)
Fixed income, available-for-sale	2,757,381	2,515,581
ABS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	155,594	173,256
Gross Unrealized Gains	2,157	1,292
Gross Unrealized Losses	(1,234)	(3,275)
Fixed income, available-for-sale	156,517	171,273
RMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	373,485	347,132
Gross Unrealized Gains	2,103	4,297
Gross Unrealized Losses	(25,528)	(20,656)
Fixed income, available-for-sale	350,060	330,773
CMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	243,840	293,204
Gross Unrealized Gains	21	5
Gross Unrealized Losses	(7,990)	(15,752)
Fixed income, available-for-sale	235,871	277,457
CLOs
Debt Securities, Available-for-sale [Line Items]
Total at end of period	199,773	327,467
Gross Unrealized Gains	286	37
Gross Unrealized Losses	0	(1,408)
Fixed income, available-for-sale	200,059	326,096
Foreign government debt
Debt Securities, Available-for-sale [Line Items]
Total at end of period	4,487	4,486
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(689)	(643)
Fixed income, available-for-sale	3,798	3,843
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Total at end of period	12,015	28,327
Gross Unrealized Gains	0	3
Gross Unrealized Losses	0	0
Fixed income, available-for-sale	$ 12,015	$ 28,330