Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Assets held by insurance regulatory requirements	$ 12,200	$ 11,800
Assets held in trust for the benefit of contractual counterparties	156,900	128,400
Fixed income securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	6,061,022	5,926,785
Gross Unrealized Gains	28,235	5,953
Gross Unrealized Losses	(366,144)	(523,040)
Fixed income, available-for-sale	5,723,113	5,409,698
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	167,995	145,581
Gross Unrealized Gains	51	2
Gross Unrealized Losses	(6,364)	(9,683)
Fixed income, available-for-sale	161,682	135,900
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,092,754	2,400,261
Gross Unrealized Gains	5,159	4,866
Gross Unrealized Losses	(189,835)	(256,073)
Fixed income, available-for-sale	1,908,078	2,149,054
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,626,401	2,416,475
Gross Unrealized Gains	17,391	1,043
Gross Unrealized Losses	(128,211)	(196,377)
Fixed income, available-for-sale	2,515,581	2,221,141
ABS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	173,256	126,723
Gross Unrealized Gains	1,292	5
Gross Unrealized Losses	(3,275)	(6,041)
Fixed income, available-for-sale	171,273	120,687
RMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	347,132	223,743
Gross Unrealized Gains	4,297	10
Gross Unrealized Losses	(20,656)	(25,744)
Fixed income, available-for-sale	330,773	198,009
CMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	293,204	257,785
Gross Unrealized Gains	5	22
Gross Unrealized Losses	(15,752)	(20,591)
Fixed income, available-for-sale	277,457	237,216
CLOs
Debt Securities, Available-for-sale [Line Items]
Total at end of period	327,467	337,656
Gross Unrealized Gains	37	5
Gross Unrealized Losses	(1,408)	(7,829)
Fixed income, available-for-sale	326,096	329,832
Foreign government debt
Debt Securities, Available-for-sale [Line Items]
Total at end of period	4,486	4,486
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(643)	(699)
Fixed income, available-for-sale	3,843	3,787
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Total at end of period	28,327	14,075
Gross Unrealized Gains	3	0
Gross Unrealized Losses	0	(3)
Fixed income, available-for-sale	$ 28,330	$ 14,072