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	Jun. 30, 2022	Dec. 31, 2021
U.S. government
Fair value
Less than 12 months	$ 142,944	$ 2,942
Total Fair Value	142,944	2,942
Amortized Cost
Amortized cost, less than 12 months	144,854	2,986
Total Cost or Amortized Cost	144,854	2,986
Unrealized Loss
Less than 12 months	(1,910)	(44)
Total Unrealized Loss	(1,910)	(44)
U.S. Agency
Fair value
Less than 12 months	15,788	1,498
Total Fair Value	15,788	1,498
Amortized Cost
Amortized cost, less than 12 months	16,625	1,515
Total Cost or Amortized Cost	16,625	1,515
Unrealized Loss
Less than 12 months	(837)	(17)
Total Unrealized Loss	(837)	(17)
Non-U.S. government & agency
Fair value
Less than 12 months	3,731	4,346
12 months and greater	2,337
Total Fair Value	6,068	4,346
Amortized Cost
Amortized cost, less than 12 months	3,797	4,500
12 months and greater	3,000
Total Cost or Amortized Cost	6,797	4,500
Unrealized Loss
Less than 12 months	(66)	(154)
12 months and greater	(663)
Total Unrealized Loss	(729)	(154)
Agency MBS
Fair value
Less than 12 months	221,563	102,145
12 months and greater	68,070	62,669
Total Fair Value	289,633	164,814
Amortized Cost
Amortized cost, less than 12 months	236,671	104,336
12 months and greater	79,471	65,429
Total Cost or Amortized Cost	316,142	169,765
Unrealized Loss
Less than 12 months	(15,108)	(2,191)
12 months and greater	(11,401)	(2,760)
Total Unrealized Loss	(26,509)	(4,951)
ABS/CMBS/MBS
Fair value
Less than 12 months	214,266	150,997
12 months and greater	22,312	3,935
Total Fair Value	236,578	154,932
Amortized Cost
Amortized cost, less than 12 months	236,298	153,235
12 months and greater	25,635	4,036
Total Cost or Amortized Cost	261,933	157,271
Unrealized Loss
Less than 12 months	(22,032)	(2,238)
12 months and greater	(3,323)	(101)
Total Unrealized Loss	(25,355)	(2,339)
Corporate Debt
Fair value
Less than 12 months	762,770	217,791
12 months and greater	78,901	53,818
Total Fair Value	841,671	271,609
Amortized Cost
Amortized cost, less than 12 months	821,145	221,010
12 months and greater	90,669	55,704
Total Cost or Amortized Cost	911,814	276,714
Unrealized Loss
Less than 12 months	(58,375)	(3,219)
12 months and greater	(11,768)	(1,886)
Total Unrealized Loss	(70,143)	(5,105)
Municipal
Fair value
Less than 12 months	387,833	162,998
12 months and greater	30,986	15,037
Total Fair Value	418,819	178,035
Amortized Cost
Amortized cost, less than 12 months	454,930	166,602
12 months and greater	41,056	15,714
Total Cost or Amortized Cost	495,986	182,316
Unrealized Loss
Less than 12 months	(67,097)	(3,604)
12 months and greater	(10,070)	(677)
Total Unrealized Loss	(77,167)	(4,281)
Fixed Income Securities
Fair value
Less than 12 months	1,748,895	642,717
12 months and greater	202,606	135,459
Total Fair Value	1,951,501	778,176
Amortized Cost
Amortized cost, less than 12 months	1,914,320	654,184
12 months and greater	239,831	140,883
Total Cost or Amortized Cost	2,154,151	795,067
Unrealized Loss
Less than 12 months	(165,425)	(11,467)
12 months and greater	(37,225)	(5,424)
Total Unrealized Loss	$ (202,650)	$ (16,891)