Investments - Schedule of Fixed Maturities Available for Sale by Component (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 18,924,914	$ 18,301,692
Allowance for Credit Losses	(7,115)	0
Gross Unrealized Gains	451,764	238,967
Gross Unrealized Losses	(1,499,357)	(2,037,294)
Fair value	$ 17,870,206	$ 16,503,365
Percentage of total fixed maturities (as a percent)	100.00%	100.00%
U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 398,450	$ 394,439
Allowance for Credit Losses	0	0
Gross Unrealized Gains	7	27
Gross Unrealized Losses	(32,306)	(38,968)
Fair value	$ 366,151	$ 355,498
Percentage of total fixed maturities (as a percent)	2.00%	2.00%
States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,296,305	$ 2,791,030
Allowance for Credit Losses	0	0
Gross Unrealized Gains	47,346	24,328
Gross Unrealized Losses	(403,329)	(505,447)
Fair value	$ 2,940,322	$ 2,309,911
Percentage of total fixed maturities (as a percent)	16.00%	14.00%
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 44,453	$ 55,164
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1	6
Gross Unrealized Losses	(10,348)	(12,706)
Fair value	$ 34,106	$ 42,464
Percentage of total fixed maturities (as a percent)	0.00%	0.00%
Corporates
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 15,062,244	$ 14,935,625
Allowance for Credit Losses	(7,115)	0	$ (387)
Gross Unrealized Gains	399,371	201,336
Gross Unrealized Losses	(1,049,317)	(1,470,897)
Fair value	$ 14,405,183	$ 13,666,064
Percentage of total fixed maturities (as a percent)	81.00%	83.00%
Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,028,151	$ 4,907,794
Allowance for Credit Losses	0	0
Gross Unrealized Gains	112,368	63,126
Gross Unrealized Losses	(388,340)	(504,489)
Fair value	$ 4,752,179	$ 4,466,431
Percentage of total fixed maturities (as a percent)	27.00%	27.00%
Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,017,967	$ 1,924,190
Allowance for Credit Losses	0	0
Gross Unrealized Gains	73,925	36,670
Gross Unrealized Losses	(94,130)	(125,713)
Fair value	$ 1,997,762	$ 1,835,147
Percentage of total fixed maturities (as a percent)	11.00%	11.00%
Corporates | Energy
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,446,480	$ 1,436,598
Allowance for Credit Losses	0	0
Gross Unrealized Gains	58,637	22,637
Gross Unrealized Losses	(62,324)	(101,923)
Fair value	$ 1,442,793	$ 1,357,312
Percentage of total fixed maturities (as a percent)	8.00%	8.00%
Corporates | Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,569,646	$ 6,667,043
Allowance for Credit Losses	(7,115)	0
Gross Unrealized Gains	154,441	78,903
Gross Unrealized Losses	(504,523)	(738,772)
Fair value	$ 6,212,449	$ 6,007,174
Percentage of total fixed maturities (as a percent)	35.00%	37.00%
Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 37,110	$ 37,098
Allowance for Credit Losses	0	0
Gross Unrealized Gains	5,036	13,266
Gross Unrealized Losses	0	0
Fair value	$ 42,146	$ 50,364
Percentage of total fixed maturities (as a percent)	0.00%	0.00%
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 86,352	$ 88,336
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3	4
Gross Unrealized Losses	(4,057)	(9,276)
Fair value	$ 82,298	$ 79,064
Percentage of total fixed maturities (as a percent)	1.00%	1.00%