Investments - Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	Mar. 31, 2023 USD ($) security	Dec. 31, 2022 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	431	858
Fair value, less than 12 months	$ 1,748,558	$ 3,463,314
Unrealized losses, less than 12 months	$ (75,311)	$ (312,320)
Number of securities, 12 months or greater | security	805	426
Fair value, 12 months or greater	$ 2,965,575	$ 1,434,379
Unrealized losses, 12 months or greater	$ (422,319)	$ (269,173)
Number of securities, total | security	1,236	1,284
Fair value, total	$ 4,714,133	$ 4,897,693
Unrealized losses, total	$ (497,630)	$ (581,493)
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	7	14
Fair value, less than 12 months	$ 35,340	$ 86,964
Unrealized losses, less than 12 months	$ (2,589)	$ (21,370)
Number of securities, 12 months or greater | security	17	10
Fair value, 12 months or greater	$ 108,647	$ 47,770
Unrealized losses, 12 months or greater	$ (21,780)	$ (7,532)
Number of securities, total | security	24	24
Fair value, total	$ 143,987	$ 134,734
Unrealized losses, total	$ (24,369)	$ (28,902)
State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	24	43
Fair value, less than 12 months	$ 84,812	$ 116,285
Unrealized losses, less than 12 months	$ (4,293)	$ (14,231)
Number of securities, 12 months or greater | security	36	20
Fair value, 12 months or greater	$ 53,004	$ 25,401
Unrealized losses, 12 months or greater	$ (12,178)	$ (7,708)
Number of securities, total | security	60	63
Fair value, total	$ 137,816	$ 141,686
Unrealized losses, total	$ (16,471)	$ (21,939)
Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	227	411
Fair value, less than 12 months	$ 980,939	$ 1,769,547
Unrealized losses, less than 12 months	$ (46,110)	$ (176,768)
Number of securities, 12 months or greater | security	362	203
Fair value, 12 months or greater	$ 1,418,761	$ 701,936
Unrealized losses, 12 months or greater	$ (257,665)	$ (173,769)
Number of securities, total | security	589	614
Fair value, total	$ 2,399,700	$ 2,471,483
Unrealized losses, total	$ (303,775)	$ (350,537)
RMBS
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	49	124
Fair value, less than 12 months	$ 319,516	$ 610,812
Unrealized losses, less than 12 months	$ (10,804)	$ (46,117)
Number of securities, 12 months or greater | security	132	59
Fair value, 12 months or greater	$ 500,482	$ 261,370
Unrealized losses, 12 months or greater	$ (71,920)	$ (52,442)
Number of securities, total | security	181	183
Fair value, total	$ 819,998	$ 872,182
Unrealized losses, total	$ (82,724)	$ (98,559)
CMBS
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	40	108
Fair value, less than 12 months	$ 134,349	$ 469,100
Unrealized losses, less than 12 months	$ (6,053)	$ (38,178)
Number of securities, 12 months or greater | security	120	55
Fair value, 12 months or greater	$ 452,445	$ 121,277
Unrealized losses, 12 months or greater	$ (41,666)	$ (14,368)
Number of securities, total | security	160	163
Fair value, total	$ 586,794	$ 590,377
Unrealized losses, total	$ (47,719)	$ (52,546)
CLO
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	35	94
Fair value, less than 12 months	$ 93,096	$ 246,705
Unrealized losses, less than 12 months	$ (4,139)	$ (10,271)
Number of securities, 12 months or greater | security	111	61
Fair value, 12 months or greater	$ 383,681	$ 245,584
Unrealized losses, 12 months or greater	$ (13,306)	$ (10,214)
Number of securities, total | security	146	155
Fair value, total	$ 476,777	$ 492,289
Unrealized losses, total	$ (17,445)	$ (20,485)
Other ABS
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	47	61
Fair value, less than 12 months	$ 89,870	$ 115,181
Unrealized losses, less than 12 months	$ (1,270)	$ (3,603)
Number of securities, 12 months or greater | security	26	18
Fair value, 12 months or greater	$ 43,541	$ 31,041
Unrealized losses, 12 months or greater	$ (3,697)	$ (3,140)
Number of securities, total | security	73	79
Fair value, total	$ 133,411	$ 146,222
Unrealized losses, total	$ (4,967)	$ (6,743)
Mortgage insurance-linked notes
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	2	2
Fair value, less than 12 months	$ 10,636	$ 43,745
Unrealized losses, less than 12 months	$ (53)	$ (1,639)
Number of securities, 12 months or greater | security	0	0
Fair value, 12 months or greater	$ 0	$ 0
Unrealized losses, 12 months or greater	$ 0	$ 0
Number of securities, total | security	2	2
Fair value, total	$ 10,636	$ 43,745
Unrealized losses, total	$ (53)	$ (1,639)
Foreign government and agency securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	0	1
Fair value, less than 12 months	$ 0	$ 4,975
Unrealized losses, less than 12 months	$ 0	$ (143)
Number of securities, 12 months or greater | security	1	0
Fair value, 12 months or greater	$ 5,014	$ 0
Unrealized losses, 12 months or greater	$ (107)	$ 0
Number of securities, total | security	1	1
Fair value, total	$ 5,014	$ 4,975
Unrealized losses, total	$ (107)	$ (143)