Investments - Gross Unrealized Losses and Related Fair Values of Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	$ 381,388	$ 3,463,314
Unrealized losses, less than 12 months	(4,567)	(312,320)
Fair value, 12 months or greater	3,981,258	1,434,379
Unrealized losses, 12 months or greater	(438,964)	(269,173)
Fair value, total	4,362,646	4,897,693
Unrealized losses, total	(443,531)	(581,493)
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	15,650	86,964
Unrealized losses, less than 12 months	(938)	(21,370)
Fair value, 12 months or greater	113,678	47,770
Unrealized losses, 12 months or greater	(25,953)	(7,532)
Fair value, total	129,328	134,734
Unrealized losses, total	(26,891)	(28,902)
State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	17,154	116,285
Unrealized losses, less than 12 months	(551)	(14,231)
Fair value, 12 months or greater	104,949	25,401
Unrealized losses, 12 months or greater	(15,486)	(7,708)
Fair value, total	122,103	141,686
Unrealized losses, total	(16,037)	(21,939)
Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	161,924	1,769,547
Unrealized losses, less than 12 months	(1,261)	(176,768)
Fair value, 12 months or greater	2,055,113	701,936
Unrealized losses, 12 months or greater	(257,916)	(173,769)
Fair value, total	2,217,037	2,471,483
Unrealized losses, total	(259,177)	(350,537)
RMBS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	113,506	610,812
Unrealized losses, less than 12 months	(1,439)	(46,117)
Fair value, 12 months or greater	653,955	261,370
Unrealized losses, 12 months or greater	(82,659)	(52,442)
Fair value, total	767,461	872,182
Unrealized losses, total	(84,098)	(98,559)
CMBS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	8,558	469,100
Unrealized losses, less than 12 months	(31)	(38,178)
Fair value, 12 months or greater	546,104	121,277
Unrealized losses, 12 months or greater	(46,666)	(14,368)
Fair value, total	554,662	590,377
Unrealized losses, total	(46,697)	(52,546)
CLO
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	15,083	246,705
Unrealized losses, less than 12 months	(25)	(10,271)
Fair value, 12 months or greater	438,294	245,584
Unrealized losses, 12 months or greater	(6,528)	(10,214)
Fair value, total	453,377	492,289
Unrealized losses, total	(6,553)	(20,485)
Other ABS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	49,513	115,181
Unrealized losses, less than 12 months	(322)	(3,603)
Fair value, 12 months or greater	64,078	31,041
Unrealized losses, 12 months or greater	(3,715)	(3,140)
Fair value, total	113,591	146,222
Unrealized losses, total	(4,037)	(6,743)
Foreign government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	0	4,975
Unrealized losses, less than 12 months	0	(143)
Fair value, 12 months or greater	5,087	0
Unrealized losses, 12 months or greater	(41)	0
Fair value, total	5,087	4,975
Unrealized losses, total	$ (41)	(143)
Mortgage insurance-linked notes
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months		43,745
Unrealized losses, less than 12 months		(1,639)
Fair value, 12 months or greater		0
Unrealized losses, 12 months or greater		0
Fair value, total		43,745
Unrealized losses, total		$ (1,639)