Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 1,254,707	$ 300,911
12 Months or Greater - Fair Value	64,345	80,269
Total - Fair Value	1,319,052	381,180
Less Than 12 Months - Unrealized Losses	(14,391)	(3,827)
12 Months or Greater - Unrealized Losses	(559)	(832)
Total - Unrealized Losses	(14,950)	(4,659)
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	88,312	2,690
12 Months or Greater - Fair Value	0	0
Total - Fair Value	88,312	2,690
Less Than 12 Months - Unrealized Losses	(277)	(2)
12 Months or Greater - Unrealized Losses	0	0
Total - Unrealized Losses	(277)	(2)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	250,099	31,416
12 Months or Greater - Fair Value	15,638	0
Total - Fair Value	265,737	31,416
Less Than 12 Months - Unrealized Losses	(3,479)	(256)
12 Months or Greater - Unrealized Losses	(160)	0
Total - Unrealized Losses	(3,639)	(256)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	468,720	44,968
12 Months or Greater - Fair Value	0	0
Total - Fair Value	468,720	44,968
Less Than 12 Months - Unrealized Losses	(8,438)	(1,728)
12 Months or Greater - Unrealized Losses	0	0
Total - Unrealized Losses	(8,438)	(1,728)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	58,901	14,929
12 Months or Greater - Fair Value	0	0
Total - Fair Value	58,901	14,929
Less Than 12 Months - Unrealized Losses	(198)	(18)
12 Months or Greater - Unrealized Losses	0	0
Total - Unrealized Losses	(198)	(18)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	174,094	98,409
12 Months or Greater - Fair Value	26,733	3,566
Total - Fair Value	200,827	101,975
Less Than 12 Months - Unrealized Losses	(1,302)	(773)
12 Months or Greater - Unrealized Losses	(364)	(65)
Total - Unrealized Losses	(1,666)	(838)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	82,653	13,212
12 Months or Greater - Fair Value	0	2,799
Total - Fair Value	82,653	16,011
Less Than 12 Months - Unrealized Losses	(628)	(789)
12 Months or Greater - Unrealized Losses	0	(336)
Total - Unrealized Losses	(628)	(1,125)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	127,483	95,287
12 Months or Greater - Fair Value	21,974	73,904
Total - Fair Value	149,457	169,191
Less Than 12 Months - Unrealized Losses	(29)	(261)
12 Months or Greater - Unrealized Losses	(35)	(431)
Total - Unrealized Losses	(64)	$ (692)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	4,445
12 Months or Greater - Fair Value	0
Total - Fair Value	4,445
Less Than 12 Months - Unrealized Losses	(40)
12 Months or Greater - Unrealized Losses	0
Total - Unrealized Losses	$ (40)