Investments - Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	Sep. 30, 2022 USD ($) security	Dec. 31, 2021 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	1,041	519
Fair value, less than 12 months	$ 3,995,493	$ 2,004,842
Unrealized losses, less than 12 months	$ (443,945)	$ (28,445)
Number of securities, 12 months or greater | security	245	60
Fair value, 12 months or greater	$ 883,682	$ 166,248
Unrealized losses, 12 months or greater	$ (192,786)	$ (9,593)
Number of securities, total | security	1,286	579
Fair value, total	$ 4,879,175	$ 2,171,090
Unrealized losses, total	$ (636,731)	$ (38,038)
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	16	14
Fair value, less than 12 months	$ 94,560	$ 101,602
Unrealized losses, less than 12 months	$ (22,377)	$ (1,165)
Number of securities, 12 months or greater | security	8	2
Fair value, 12 months or greater	$ 48,322	$ 6,937
Unrealized losses, 12 months or greater	$ (6,134)	$ (231)
Number of securities, total | security	24	16
Fair value, total	$ 142,882	$ 108,539
Unrealized losses, total	$ (28,511)	$ (1,396)
State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	57	20
Fair value, less than 12 months	$ 133,696	$ 32,721
Unrealized losses, less than 12 months	$ (19,265)	$ (401)
Number of securities, 12 months or greater | security	7	0
Fair value, 12 months or greater	$ 8,747	$ 0
Unrealized losses, 12 months or greater	$ (2,037)	$ 0
Number of securities, total | security	64	20
Fair value, total	$ 142,443	$ 32,721
Unrealized losses, total	$ (21,302)	$ (401)
Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	487	209
Fair value, less than 12 months	$ 1,992,250	$ 864,355
Unrealized losses, less than 12 months	$ (266,626)	$ (16,799)
Number of securities, 12 months or greater | security	123	34
Fair value, 12 months or greater	$ 451,971	$ 99,475
Unrealized losses, 12 months or greater	$ (133,946)	$ (8,087)
Number of securities, total | security	610	243
Fair value, total	$ 2,444,221	$ 963,830
Unrealized losses, total	$ (400,572)	$ (24,886)
RMBS
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	152	57
Fair value, less than 12 months	$ 716,471	$ 365,476
Unrealized losses, less than 12 months	$ (80,207)	$ (6,749)
Number of securities, 12 months or greater | security	24	3
Fair value, 12 months or greater	$ 130,465	$ 1,543
Unrealized losses, 12 months or greater	$ (30,177)	$ (28)
Number of securities, total | security	176	60
Fair value, total	$ 846,936	$ 367,019
Unrealized losses, total	$ (110,384)	$ (6,777)
CMBS
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	129	81
Fair value, less than 12 months	$ 517,151	$ 188,457
Unrealized losses, less than 12 months	$ (35,210)	$ (2,053)
Number of securities, 12 months or greater | security	39	9
Fair value, 12 months or greater	$ 86,743	$ 22,050
Unrealized losses, 12 months or greater	$ (11,986)	$ (1,015)
Number of securities, total | security	168	90
Fair value, total	$ 603,894	$ 210,507
Unrealized losses, total	$ (47,196)	$ (3,068)
CLO
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	121	84
Fair value, less than 12 months	$ 359,365	$ 313,380
Unrealized losses, less than 12 months	$ (14,230)	$ (675)
Number of securities, 12 months or greater | security	36	11
Fair value, 12 months or greater	$ 138,584	$ 35,612
Unrealized losses, 12 months or greater	$ (6,082)	$ (223)
Number of securities, total | security	157	95
Fair value, total	$ 497,949	$ 348,992
Unrealized losses, total	$ (20,312)	$ (898)
Other ABS
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	76	54
Fair value, less than 12 months	$ 133,593	$ 138,851
Unrealized losses, less than 12 months	$ (3,938)	$ (603)
Number of securities, 12 months or greater | security	8	1
Fair value, 12 months or greater	$ 18,850	$ 631
Unrealized losses, 12 months or greater	$ (2,424)	$ (9)
Number of securities, total | security	84	55
Fair value, total	$ 152,443	$ 139,482
Unrealized losses, total	$ (6,362)	$ (612)
Mortgage insurance-linked notes
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	2
Fair value, less than 12 months	$ 43,437
Unrealized losses, less than 12 months	$ (1,947)
Number of securities, 12 months or greater | security	0
Fair value, 12 months or greater	$ 0
Unrealized losses, 12 months or greater	$ 0
Number of securities, total | security	2
Fair value, total	$ 43,437
Unrealized losses, total	$ (1,947)
Foreign government and agency securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	1
Fair value, less than 12 months	$ 4,970
Unrealized losses, less than 12 months	$ (145)
Number of securities, 12 months or greater | security	0
Fair value, 12 months or greater	$ 0
Unrealized losses, 12 months or greater	$ 0
Number of securities, total | security	1
Fair value, total	$ 4,970
Unrealized losses, total	$ (145)