Investments - Schedule of Fixed Maturities Available for Sale by Component (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 18,948,357		$ 18,835,809
Allowance for Credit Losses	(10,415)	$ (10,355)	(10,395)	$ (7,132)	$ (7,132)	$ (7,115)
Gross Unrealized Gains	448,518		254,587
Gross Unrealized Losses	(1,589,706)		(1,924,989)
Fair value	$ 17,796,754		$ 17,155,012
Percentage of total fixed maturities	100.00%		100.00%
U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 405,807		$ 401,753
Allowance for Credit Losses	0		0
Gross Unrealized Gains	147		1
Gross Unrealized Losses	(26,787)		(42,794)
Fair value	$ 379,167		$ 358,960
Percentage of total fixed maturities	2.00%		2.00%
States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,390,124		$ 3,300,901
Allowance for Credit Losses	0		0
Gross Unrealized Gains	28,759		20,662
Gross Unrealized Losses	(545,729)		(534,759)
Fair value	$ 2,873,154		$ 2,786,804
Percentage of total fixed maturities	16.00%		16.00%
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 47,915		$ 36,883
Allowance for Credit Losses	0		0
Gross Unrealized Gains	291		18
Gross Unrealized Losses	(8,195)		(8,870)
Fair value	$ 40,011		$ 28,031
Percentage of total fixed maturities	0.00%		0.00%
Corporates
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 15,010,998		$ 14,976,815
Allowance for Credit Losses	(7,118)		(7,098)
Gross Unrealized Gains	418,938		227,924
Gross Unrealized Losses	(1,008,042)		(1,336,380)
Fair value	$ 14,414,776		$ 13,861,261
Percentage of total fixed maturities	81.00%		81.00%
Corporates | Industrials
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 7,891,877		$ 7,889,074
Allowance for Credit Losses	(7,118)		(7,098)
Gross Unrealized Gains	193,171		105,610
Gross Unrealized Losses	(601,835)		(805,330)
Fair value	$ 7,476,095		$ 7,182,256
Percentage of total fixed maturities	42.00%		42.00%
Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,018,604		$ 5,006,375
Allowance for Credit Losses	0		0
Gross Unrealized Gains	146,863		82,598
Gross Unrealized Losses	(318,860)		(413,043)
Fair value	$ 4,846,607		$ 4,675,930
Percentage of total fixed maturities	27.00%		27.00%
Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,100,517		$ 2,081,366
Allowance for Credit Losses	0		0
Gross Unrealized Gains	78,904		39,716
Gross Unrealized Losses	(87,347)		(118,007)
Fair value	$ 2,092,074		$ 2,003,075
Percentage of total fixed maturities	12.00%		12.00%
Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 0		$ 36,923
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		5,943
Gross Unrealized Losses	0		0
Fair value	$ 0		$ 42,866
Percentage of total fixed maturities	0.00%		0.00%
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 93,513		$ 82,534
Allowance for Credit Losses	(3,297)		(3,297)
Gross Unrealized Gains	383		39
Gross Unrealized Losses	(953)		(2,186)
Fair value	$ 89,646		$ 77,090
Percentage of total fixed maturities	1.00%		1.00%