Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 659,770		$ 624,334
12 Months or Greater - Fair Value	154,764		379,411
Total - Fair Value	814,534		1,003,745
Less Than 12 Months - Unrealized Losses	(6,805)		(5,426)
12 Months or Greater - Unrealized Losses	(1,962)		(4,476)
Total - Unrealized Losses	(8,767)		(9,902)
Allowance for Expected Credit Loss	362
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	5,004		57,301
12 Months or Greater - Fair Value	0		5,806
Total - Fair Value	5,004		63,107
Less Than 12 Months - Unrealized Losses	(15)		(200)
12 Months or Greater - Unrealized Losses	0		(10)
Total - Unrealized Losses	(15)		(210)
Allowance for Expected Credit Loss	0
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	72,620		74,859
12 Months or Greater - Fair Value	0		6,957
Total - Fair Value	72,620		81,816
Less Than 12 Months - Unrealized Losses	(891)		(847)
12 Months or Greater - Unrealized Losses	0		(10)
Total - Unrealized Losses	(891)		(857)
Allowance for Expected Credit Loss	0
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	263,665		221,357
12 Months or Greater - Fair Value	0		43,505
Total - Fair Value	263,665		264,862
Less Than 12 Months - Unrealized Losses	(2,748)		(2,847)
12 Months or Greater - Unrealized Losses	0		(161)
Total - Unrealized Losses	(2,748)		(3,008)
Allowance for Expected Credit Loss	0
Asset backed securities (“ABS”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	17,505		21,542
12 Months or Greater - Fair Value	0		3,851
Total - Fair Value	17,505		25,393
Less Than 12 Months - Unrealized Losses	(145)		(118)
12 Months or Greater - Unrealized Losses	0		(60)
Total - Unrealized Losses	(145)		(178)
Allowance for Expected Credit Loss	362	$ 0	0
Residential mortgage backed securities (“RMBS”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	128,358		105,443
12 Months or Greater - Fair Value	3,886		110,452
Total - Fair Value	132,244		215,895
Less Than 12 Months - Unrealized Losses	(1,028)		(461)
12 Months or Greater - Unrealized Losses	(119)		(2,766)
Total - Unrealized Losses	(1,147)		(3,227)
Allowance for Expected Credit Loss	0
Commercial mortgage backed securities (“CMBS”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	13,065		62,388
12 Months or Greater - Fair Value	1,338		11,852
Total - Fair Value	14,403		74,240
Less Than 12 Months - Unrealized Losses	(707)		(728)
12 Months or Greater - Unrealized Losses	(139)		(51)
Total - Unrealized Losses	(846)		(779)
Allowance for Expected Credit Loss	0
Collateralized loan obligations (“CLOs”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	159,553		81,444
12 Months or Greater - Fair Value	149,540		196,988
Total - Fair Value	309,093		278,432
Less Than 12 Months - Unrealized Losses	(1,271)		(225)
12 Months or Greater - Unrealized Losses	(1,704)		(1,418)
Total - Unrealized Losses	(2,975)		$ (1,643)
Allowance for Expected Credit Loss	$ 0