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, 2021	Dec. 31, 2020
U.S. government
Fair Value
Less than 12 months	$ 4,864	$ 5,680
Total Fair Value	4,864	5,680
Amortized Cost
Amortized cost, less than 12 months	5,937	5,937
Total Cost or Amortized Cost	5,937	5,937
Unrealized Loss
Less than 12 months	(1,073)	(257)
Total Unrealized Loss	(1,073)	(257)
Non-U.S. government & agency
Fair Value
Less than 12 months	2,852
Total Fair Value	2,852
Amortized Cost
Amortized cost, less than 12 months	3,000
Total Cost or Amortized Cost	3,000
Unrealized Loss
Less than 12 months	(148)
Total Unrealized Loss	(148)
Agency MBS
Fair Value
Less than 12 months	92,446	43,999
Total Fair Value	92,446	43,999
Amortized Cost
Amortized cost, less than 12 months	95,465	44,732
Total Cost or Amortized Cost	95,465	44,732
Unrealized Loss
Less than 12 months	(3,019)	(733)
Total Unrealized Loss	(3,019)	(733)
ABS/CMBS/MBS
Fair Value
Less than 12 months	55,703	32,771
12 months and greater	7,235	16,161
Total Fair Value	62,938	48,932
Amortized Cost
Amortized cost, less than 12 months	56,341	33,094
12 months and greater	7,255	16,251
Total Cost or Amortized Cost	63,596	49,345
Unrealized Loss
Less than 12 months	(638)	(323)
12 months and greater	(20)	(90)
Total Unrealized Loss	(658)	(413)
Corporate Debt
Fair Value
Less than 12 months	165,717	52,655
12 months and greater	14,193	6,235
Total Fair Value	179,910	58,890
Amortized Cost
Amortized cost, less than 12 months	170,204	53,440
12 months and greater	14,370	6,383
Total Cost or Amortized Cost	184,574	59,823
Unrealized Loss
Less than 12 months	(4,487)	(785)
12 months and greater	(177)	(148)
Total Unrealized Loss	(4,664)	(933)
Municipal
Fair Value
Less than 12 months	115,596	25,676
Total Fair Value	115,596	25,676
Amortized Cost
Amortized cost, less than 12 months	121,072	25,894
Total Cost or Amortized Cost	121,072	25,894
Unrealized Loss
Less than 12 months	(5,476)	(218)
Total Unrealized Loss	(5,476)	(218)
Debt Securities
Fair Value
Less than 12 months	437,178	160,781
12 months and greater	21,428	22,396
Total Fair Value	458,606	183,177
Amortized Cost
Amortized cost, less than 12 months	452,019	163,097
12 months and greater	21,625	22,634
Total Cost or Amortized Cost	473,644	185,731
Unrealized Loss
Less than 12 months	(14,841)	(2,316)
12 months and greater	(197)	(238)
Total Unrealized Loss	$ (15,038)	$ (2,554)