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	Sep. 30, 2021	Dec. 31, 2020
U.S. government
Fair Value
Less than 12 months	$ 2,976	$ 5,680
12 months and greater	5,152
Total Fair Value	8,128	5,680
Amortized Cost
Amortized cost, less than 12 months	2,986	5,937
12 months and greater	5,939
Total Cost or Amortized Cost	8,925	5,937
Unrealized Loss
Less than 12 months	(10)	(257)
12 months and greater	(787)
Total Unrealized Loss	(797)	(257)
U.S. Agency
Fair Value
Less than 12 months	1,003
Total Fair Value	1,003
Amortized Cost
Amortized cost, less than 12 months	1,003
Total Cost or Amortized Cost	1,003
Non-U.S. government & agency
Fair Value
Less than 12 months	2,817
Total Fair Value	2,817
Amortized Cost
Amortized cost, less than 12 months	3,000
Total Cost or Amortized Cost	3,000
Unrealized Loss
Less than 12 months	(183)
Total Unrealized Loss	(183)
Agency MBS
Fair Value
Less than 12 months	128,783	43,999
12 months and greater	15,940
Total Fair Value	144,723	43,999
Amortized Cost
Amortized cost, less than 12 months	131,277	44,732
12 months and greater	16,755
Total Cost or Amortized Cost	148,032	44,732
Unrealized Loss
Less than 12 months	(2,494)	(733)
12 months and greater	(815)
Total Unrealized Loss	(3,309)	(733)
ABS/CMBS/MBS
Fair Value
Less than 12 months	92,083	32,771
12 months and greater	3,832	16,161
Total Fair Value	95,915	48,932
Amortized Cost
Amortized cost, less than 12 months	93,020	33,094
12 months and greater	3,850	16,251
Total Cost or Amortized Cost	96,870	49,345
Unrealized Loss
Less than 12 months	(937)	(323)
12 months and greater	(18)	(90)
Total Unrealized Loss	(955)	(413)
Corporate Debt
Fair Value
Less than 12 months	173,508	52,655
12 months and greater	18,574	6,235
Total Fair Value	192,082	58,890
Amortized Cost
Amortized cost, less than 12 months	175,695	53,440
12 months and greater	19,299	6,383
Total Cost or Amortized Cost	194,994	59,823
Unrealized Loss
Less than 12 months	(2,187)	(785)
12 months and greater	(725)	(148)
Total Unrealized Loss	(2,912)	(933)
Municipal
Fair Value
Less than 12 months	154,883	25,676
12 months and greater	11,179
Total Fair Value	166,062	25,676
Amortized Cost
Amortized cost, less than 12 months	158,310	25,894
12 months and greater	11,669
Total Cost or Amortized Cost	169,979	25,894
Unrealized Loss
Less than 12 months	(3,427)	(218)
12 months and greater	(490)
Total Unrealized Loss	(3,917)	(218)
Debt Securities
Fair Value
Less than 12 months	556,053	160,781
12 months and greater	54,677	22,396
Total Fair Value	610,730	183,177
Amortized Cost
Amortized cost, less than 12 months	565,291	163,097
12 months and greater	57,512	22,634
Total Cost or Amortized Cost	622,803	185,731
Unrealized Loss
Less than 12 months	(9,238)	(2,316)
12 months and greater	(2,835)	(238)
Total Unrealized Loss	$ (12,073)	$ (2,554)