Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 3,219,080	$ 1,970,111
12 Months or Greater - Fair Value	322,775	85,830
Total - Fair Value	3,541,855	2,055,941
Less Than 12 Months - Unrealized Losses	(178,797)	(26,842)
12 Months or Greater - Unrealized Losses	(27,366)	(1,625)
Total - Unrealized Losses	(206,163)	(28,467)
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	89,952	91,154
12 Months or Greater - Fair Value	2,589	2,616
Total - Fair Value	92,541	93,770
Less Than 12 Months - Unrealized Losses	(4,508)	(790)
12 Months or Greater - Unrealized Losses	(204)	(78)
Total - Unrealized Losses	(4,712)	(868)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	1,033,010	452,021
12 Months or Greater - Fair Value	36,513	15,540
Total - Fair Value	1,069,523	467,561
Less Than 12 Months - Unrealized Losses	(86,569)	(7,189)
12 Months or Greater - Unrealized Losses	(3,677)	(207)
Total - Unrealized Losses	(90,246)	(7,396)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	1,305,092	865,085
12 Months or Greater - Fair Value	168,499	10,997
Total - Fair Value	1,473,591	876,082
Less Than 12 Months - Unrealized Losses	(66,461)	(13,260)
12 Months or Greater - Unrealized Losses	(18,199)	(516)
Total - Unrealized Losses	(84,660)	(13,776)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	82,000	100,064
12 Months or Greater - Fair Value	8,660	1,552
Total - Fair Value	90,660	101,616
Less Than 12 Months - Unrealized Losses	(3,066)	(998)
12 Months or Greater - Unrealized Losses	(398)	(10)
Total - Unrealized Losses	(3,464)	(1,008)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	204,868	180,586
12 Months or Greater - Fair Value	39,473	31,641
Total - Fair Value	244,341	212,227
Less Than 12 Months - Unrealized Losses	(8,805)	(2,548)
12 Months or Greater - Unrealized Losses	(2,789)	(730)
Total - Unrealized Losses	(11,594)	(3,278)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	222,998	89,889
12 Months or Greater - Fair Value	25,201	1,511
Total - Fair Value	248,199	91,400
Less Than 12 Months - Unrealized Losses	(7,773)	(1,887)
12 Months or Greater - Unrealized Losses	(1,667)	(49)
Total - Unrealized Losses	(9,440)	(1,936)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	277,052	177,663
12 Months or Greater - Fair Value	41,840	21,973
Total - Fair Value	318,892	199,636
Less Than 12 Months - Unrealized Losses	(1,238)	(71)
12 Months or Greater - Unrealized Losses	(432)	(35)
Total - Unrealized Losses	(1,670)	(106)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	4,108	13,649
12 Months or Greater - Fair Value	0	0
Total - Fair Value	4,108	13,649
Less Than 12 Months - Unrealized Losses	(377)	(99)
12 Months or Greater - Unrealized Losses	0	0
Total - Unrealized Losses	$ (377)	$ (99)