Investments - Schedule of Securities in an Unrealized Loss Position Segregated by Type and Length of Time in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
U.S. government
Fair value
Less than 12 months	$ 35,025	$ 2,942
Total Fair Value	35,025	2,942
Amortized Cost
Amortized cost, less than 12 months	35,535	2,986
Total Cost or Amortized Cost	35,535	2,986
Unrealized Loss
Less than 12 months	(510)	(44)
Total Unrealized Loss	(510)	(44)
U.S. Agency
Fair value
Less than 12 months	6,693	1,498
Total Fair Value	6,693	1,498
Amortized Cost
Amortized cost, less than 12 months	7,028	1,515
Total Cost or Amortized Cost	7,028	1,515
Unrealized Loss
Less than 12 months	(335)	(17)
Total Unrealized Loss	(335)	(17)
Non-U.S. government & agency
Fair value
Less than 12 months		4,346
12 months and greater	2,653
Total Fair Value	2,653	4,346
Amortized Cost
Amortized cost, less than 12 months		4,500
12 months and greater	3,000
Total Cost or Amortized Cost	3,000	4,500
Unrealized Loss
Less than 12 months		(154)
12 months and greater	(347)
Total Unrealized Loss	(347)	(154)
Agency MBS
Fair value
Less than 12 months	190,950	102,145
12 months and greater	59,572	62,669
Total Fair Value	250,522	164,814
Amortized Cost
Amortized cost, less than 12 months	200,176	104,336
12 months and greater	66,038	65,429
Total Cost or Amortized Cost	266,214	169,765
Unrealized Loss
Less than 12 months	(9,226)	(2,191)
12 months and greater	(6,466)	(2,760)
Total Unrealized Loss	(15,692)	(4,951)
ABS/CMBS/MBS
Fair value
Less than 12 months	218,189	150,997
12 months and greater	19,448	3,935
Total Fair Value	237,637	154,932
Amortized Cost
Amortized cost, less than 12 months	230,803	153,235
12 months and greater	20,685	4,036
Total Cost or Amortized Cost	251,488	157,271
Unrealized Loss
Less than 12 months	(12,614)	(2,238)
12 months and greater	(1,237)	(101)
Total Unrealized Loss	(13,851)	(2,339)
Corporate Debt
Fair value
Less than 12 months	460,342	217,791
12 months and greater	68,905	53,818
Total Fair Value	529,247	271,609
Amortized Cost
Amortized cost, less than 12 months	485,129	221,010
12 months and greater	75,475	55,704
Total Cost or Amortized Cost	560,604	276,714
Unrealized Loss
Less than 12 months	(24,787)	(3,219)
12 months and greater	(6,570)	(1,886)
Total Unrealized Loss	(31,357)	(5,105)
Municipal
Fair value
Less than 12 months	350,366	162,998
12 months and greater	32,880	15,037
Total Fair Value	383,246	178,035
Amortized Cost
Amortized cost, less than 12 months	380,619	166,602
12 months and greater	38,521	15,714
Total Cost or Amortized Cost	419,140	182,316
Unrealized Loss
Less than 12 months	(30,253)	(3,604)
12 months and greater	(5,641)	(677)
Total Unrealized Loss	(35,894)	(4,281)
Fixed Income Securities
Fair value
Less than 12 months	1,261,565	642,717
12 months and greater	183,458	135,459
Total Fair Value	1,445,023	778,176
Amortized Cost
Amortized cost, less than 12 months	1,339,290	654,184
12 months and greater	203,719	140,883
Total Cost or Amortized Cost	1,543,009	795,067
Unrealized Loss
Less than 12 months	(77,725)	(11,467)
12 months and greater	(20,261)	(5,424)
Total Unrealized Loss	$ (97,986)	$ (16,891)