Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 1,045,439	$ 300,911
12 Months or Greater - Fair Value	36,029	80,269
Total - Fair Value	1,081,468	381,180
Less Than 12 Months - Unrealized Losses	(11,473)	(3,827)
12 Months or Greater - Unrealized Losses	(231)	(832)
Total - Unrealized Losses	(11,704)	(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	68,299	2,690
12 Months or Greater - Fair Value	0	0
Total - Fair Value	68,299	2,690
Less Than 12 Months - Unrealized Losses	(149)	(2)
12 Months or Greater - Unrealized Losses	0	0
Total - Unrealized Losses	(149)	(2)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	153,214	31,416
12 Months or Greater - Fair Value	0	0
Total - Fair Value	153,214	31,416
Less Than 12 Months - Unrealized Losses	(2,087)	(256)
12 Months or Greater - Unrealized Losses	0	0
Total - Unrealized Losses	(2,087)	(256)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	358,140	44,968
12 Months or Greater - Fair Value	0	0
Total - Fair Value	358,140	44,968
Less Than 12 Months - Unrealized Losses	(5,966)	(1,728)
12 Months or Greater - Unrealized Losses	0	0
Total - Unrealized Losses	(5,966)	(1,728)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	32,548	14,929
12 Months or Greater - Fair Value	0	0
Total - Fair Value	32,548	14,929
Less Than 12 Months - Unrealized Losses	(101)	(18)
12 Months or Greater - Unrealized Losses	0	0
Total - Unrealized Losses	(101)	(18)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	222,192	98,409
12 Months or Greater - Fair Value	10,056	3,566
Total - Fair Value	232,248	101,975
Less Than 12 Months - Unrealized Losses	(2,271)	(773)
12 Months or Greater - Unrealized Losses	(125)	(65)
Total - Unrealized Losses	(2,396)	(838)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	81,828	13,212
12 Months or Greater - Fair Value	4,000	2,799
Total - Fair Value	85,828	16,011
Less Than 12 Months - Unrealized Losses	(866)	(789)
12 Months or Greater - Unrealized Losses	(68)	(336)
Total - Unrealized Losses	(934)	(1,125)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	124,739	95,287
12 Months or Greater - Fair Value	21,973	73,904
Total - Fair Value	146,712	169,191
Less Than 12 Months - Unrealized Losses	(27)	(261)
12 Months or Greater - Unrealized Losses	(38)	(431)
Total - Unrealized Losses	(65)	$ (692)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	4,479
12 Months or Greater - Fair Value	0
Total - Fair Value	4,479
Less Than 12 Months - Unrealized Losses	(6)
12 Months or Greater - Unrealized Losses	0
Total - Unrealized Losses	$ (6)