Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2022 USD ($) security	Dec. 31, 2021 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 4,652,327	$ 1,970,111
12 Months or Greater - Fair Value	738,044	85,830
Total - Fair Value	5,390,371	2,055,941
Less Than 12 Months - Unrealized Losses	(439,144)	(26,842)
12 Months or Greater - Unrealized Losses	(132,240)	(1,625)
Total - Unrealized Losses	$ (571,384)	$ (28,467)
Number of securities in unrealized loss position (in securities) | security	1,346	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	$ 91,057	$ 91,154
12 Months or Greater - Fair Value	52,868	2,616
Total - Fair Value	143,925	93,770
Less Than 12 Months - Unrealized Losses	(4,565)	(790)
12 Months or Greater - Unrealized Losses	(5,729)	(78)
Total - Unrealized Losses	(10,294)	(868)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	1,993,265	452,021
12 Months or Greater - Fair Value	122,240	15,540
Total - Fair Value	2,115,505	467,561
Less Than 12 Months - Unrealized Losses	(229,380)	(7,189)
12 Months or Greater - Unrealized Losses	(35,787)	(207)
Total - Unrealized Losses	(265,167)	(7,396)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	1,897,128	865,085
12 Months or Greater - Fair Value	307,124	10,997
Total - Fair Value	2,204,252	876,082
Less Than 12 Months - Unrealized Losses	(164,708)	(13,260)
12 Months or Greater - Unrealized Losses	(62,701)	(516)
Total - Unrealized Losses	(227,409)	(13,776)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	96,910	100,064
12 Months or Greater - Fair Value	32,673	1,552
Total - Fair Value	129,583	101,616
Less Than 12 Months - Unrealized Losses	(3,741)	(998)
12 Months or Greater - Unrealized Losses	(2,078)	(10)
Total - Unrealized Losses	(5,819)	(1,008)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	124,775	180,586
12 Months or Greater - Fair Value	93,453	31,641
Total - Fair Value	218,228	212,227
Less Than 12 Months - Unrealized Losses	(13,598)	(2,548)
12 Months or Greater - Unrealized Losses	(14,774)	(730)
Total - Unrealized Losses	(28,372)	(3,278)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	183,053	89,889
12 Months or Greater - Fair Value	66,755	1,511
Total - Fair Value	249,808	91,400
Less Than 12 Months - Unrealized Losses	(13,797)	(1,887)
12 Months or Greater - Unrealized Losses	(7,283)	(49)
Total - Unrealized Losses	(21,080)	(1,936)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	266,139	177,663
12 Months or Greater - Fair Value	59,359	21,973
Total - Fair Value	325,498	199,636
Less Than 12 Months - Unrealized Losses	(9,355)	(71)
12 Months or Greater - Unrealized Losses	(2,974)	(35)
Total - Unrealized Losses	(12,329)	(106)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	13,649
12 Months or Greater - Fair Value	3,572	0
Total - Fair Value	3,572	13,649
Less Than 12 Months - Unrealized Losses	0	(99)
12 Months or Greater - Unrealized Losses	(914)	0
Total - Unrealized Losses	$ (914)	$ (99)