Investments - Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	Mar. 31, 2022 USD ($) security	Dec. 31, 2021 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	847	519
Fair value, less than 12 months	$ 3,240,895	$ 2,004,842
Unrealized losses, less than 12 months	$ (147,965)	$ (28,445)
Number of securities, 12 months or greater | security	127	60
Fair value, 12 months or greater	$ 455,614	$ 166,248
Unrealized losses, 12 months or greater	$ (55,785)	$ (9,593)
Number of securities, total | security	974	579
Fair value, total	$ 3,696,509	$ 2,171,090
Unrealized losses, total	$ (203,750)	$ (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	$ 133,485	$ 101,602
Unrealized losses, less than 12 months	$ (8,988)	$ (1,165)
Number of securities, 12 months or greater | security	5	2
Fair value, 12 months or greater	$ 20,254	$ 6,937
Unrealized losses, 12 months or greater	$ (1,107)	$ (231)
Number of securities, total | security	21	16
Fair value, total	$ 153,739	$ 108,539
Unrealized losses, total	$ (10,095)	$ (1,396)
State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	37	20
Fair value, less than 12 months	$ 61,712	$ 32,721
Unrealized losses, less than 12 months	$ (5,806)	$ (401)
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	37	20
Fair value, total	$ 61,712	$ 32,721
Unrealized losses, total	$ (5,806)	$ (401)
Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	361	209
Fair value, less than 12 months	$ 1,504,271	$ 864,355
Unrealized losses, less than 12 months	$ (94,813)	$ (16,799)
Number of securities, 12 months or greater | security	71	34
Fair value, 12 months or greater	$ 220,194	$ 99,475
Unrealized losses, 12 months or greater	$ (37,892)	$ (8,087)
Number of securities, total | security	432	243
Fair value, total	$ 1,724,465	$ 963,830
Unrealized losses, total	$ (132,705)	$ (24,886)
RMBS
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	114	57
Fair value, less than 12 months	$ 427,374	$ 365,476
Unrealized losses, less than 12 months	$ (21,712)	$ (6,749)
Number of securities, 12 months or greater | security	16	3
Fair value, 12 months or greater	$ 116,182	$ 1,543
Unrealized losses, 12 months or greater	$ (11,163)	$ (28)
Number of securities, total | security	130	60
Fair value, total	$ 543,556	$ 367,019
Unrealized losses, total	$ (32,875)	$ (6,777)
CMBS
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	135	81
Fair value, less than 12 months	$ 510,675	$ 188,457
Unrealized losses, less than 12 months	$ (9,682)	$ (2,053)
Number of securities, 12 months or greater | security	18	9
Fair value, 12 months or greater	$ 44,834	$ 22,050
Unrealized losses, 12 months or greater	$ (4,913)	$ (1,015)
Number of securities, total | security	153	90
Fair value, total	$ 555,509	$ 210,507
Unrealized losses, total	$ (14,595)	$ (3,068)
CLO
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	120	84
Fair value, less than 12 months	$ 403,176	$ 313,380
Unrealized losses, less than 12 months	$ (3,975)	$ (675)
Number of securities, 12 months or greater | security	14	11
Fair value, 12 months or greater	$ 53,121	$ 35,612
Unrealized losses, 12 months or greater	$ (689)	$ (223)
Number of securities, total | security	134	95
Fair value, total	$ 456,297	$ 348,992
Unrealized losses, total	$ (4,664)	$ (898)
Other ABS
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	61	54
Fair value, less than 12 months	$ 140,177	$ 138,851
Unrealized losses, less than 12 months	$ (2,573)	$ (603)
Number of securities, 12 months or greater | security	3	1
Fair value, 12 months or greater	$ 1,029	$ 631
Unrealized losses, 12 months or greater	$ (21)	$ (9)
Number of securities, total | security	64	55
Fair value, total	$ 141,206	$ 139,482
Unrealized losses, total	$ (2,594)	$ (612)
Mortgage insurance-linked notes
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	3
Fair value, less than 12 months	$ 60,025
Unrealized losses, less than 12 months	$ (416)
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	3
Fair value, total	$ 60,025
Unrealized losses, total	$ (416)