Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2022 USD ($) security	Dec. 31, 2021 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 4,342,497	$ 1,970,111
12 Months or Greater - Fair Value	393,966	85,830
Total - Fair Value	4,736,463	2,055,941
Less Than 12 Months - Unrealized Losses	(340,662)	(26,842)
12 Months or Greater - Unrealized Losses	(46,964)	(1,625)
Total - Unrealized Losses	$ (387,626)	$ (28,467)
Number of securities in unrealized loss position (in securities) | security	1,164	610
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 106,464	$ 91,154
12 Months or Greater - Fair Value	2,561	2,616
Total - Fair Value	109,025	93,770
Less Than 12 Months - Unrealized Losses	(6,062)	(790)
12 Months or Greater - Unrealized Losses	(233)	(78)
Total - Unrealized Losses	(6,295)	(868)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	1,471,282	452,021
12 Months or Greater - Fair Value	34,851	15,540
Total - Fair Value	1,506,133	467,561
Less Than 12 Months - Unrealized Losses	(168,115)	(7,189)
12 Months or Greater - Unrealized Losses	(5,978)	(207)
Total - Unrealized Losses	(174,093)	(7,396)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	2,043,621	865,085
12 Months or Greater - Fair Value	172,444	10,997
Total - Fair Value	2,216,065	876,082
Less Than 12 Months - Unrealized Losses	(130,168)	(13,260)
12 Months or Greater - Unrealized Losses	(27,987)	(516)
Total - Unrealized Losses	(158,155)	(13,776)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	79,046	100,064
12 Months or Greater - Fair Value	17,173	1,552
Total - Fair Value	96,219	101,616
Less Than 12 Months - Unrealized Losses	(4,066)	(998)
12 Months or Greater - Unrealized Losses	(739)	(10)
Total - Unrealized Losses	(4,805)	(1,008)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	140,146	180,586
12 Months or Greater - Fair Value	81,398	31,641
Total - Fair Value	221,544	212,227
Less Than 12 Months - Unrealized Losses	(10,365)	(2,548)
12 Months or Greater - Unrealized Losses	(7,226)	(730)
Total - Unrealized Losses	(17,591)	(3,278)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	228,340	89,889
12 Months or Greater - Fair Value	33,890	1,511
Total - Fair Value	262,230	91,400
Less Than 12 Months - Unrealized Losses	(12,783)	(1,887)
12 Months or Greater - Unrealized Losses	(2,746)	(49)
Total - Unrealized Losses	(15,529)	(1,936)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	269,852	177,663
12 Months or Greater - Fair Value	51,649	21,973
Total - Fair Value	321,501	199,636
Less Than 12 Months - Unrealized Losses	(8,363)	(71)
12 Months or Greater - Unrealized Losses	(2,055)	(35)
Total - Unrealized Losses	(10,418)	(106)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	3,746	13,649
12 Months or Greater - Fair Value	0	0
Total - Fair Value	3,746	13,649
Less Than 12 Months - Unrealized Losses	(740)	(99)
12 Months or Greater - Unrealized Losses	0	0
Total - Unrealized Losses	$ (740)	$ (99)