Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,130,009	$ 5,926,785
Fair Value	5,581,537	5,409,698
Short term fixed income securities investments held by company	177,400	67,000
Assets held by insurance regulatory requirements	11,500	11,800
Assets held in trust for the benefit of contractual counterparties	143,800	128,400
Total fixed income securities (1)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,130,009	5,926,785
Gross Unrealized Gains	1,829	5,953
Gross Unrealized Losses	(550,301)	(523,040)
Fair Value	5,581,537	5,409,698
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	220,592	145,581
Gross Unrealized Gains	7	2
Gross Unrealized Losses	(9,765)	(9,683)
Fair Value	210,834	135,900
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,178,573	2,400,261
Gross Unrealized Gains	1,234	4,866
Gross Unrealized Losses	(274,157)	(256,073)
Fair Value	1,905,650	2,149,054
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,564,817	2,416,475
Gross Unrealized Gains	498	1,043
Gross Unrealized Losses	(200,650)	(196,377)
Fair Value	2,364,665	2,221,141
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	172,262	126,723
Gross Unrealized Gains	9	5
Gross Unrealized Losses	(5,051)	(6,041)
Fair Value	167,220	120,687
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	307,402	223,743
Gross Unrealized Gains	7	10
Gross Unrealized Losses	(34,727)	(25,744)
Fair Value	272,682	198,009
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	297,091	257,785
Gross Unrealized Gains	65	22
Gross Unrealized Losses	(22,836)	(20,591)
Fair Value	274,320	237,216
CLOs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	333,974	337,656
Gross Unrealized Gains	9	5
Gross Unrealized Losses	(2,256)	(7,829)
Fair Value	331,727	329,832
Foreign government debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,486	4,486
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(859)	(699)
Fair Value	3,627	3,787
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,812	14,075
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(3)
Fair Value	$ 50,812	$ 14,072