Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 680,832	$ 624,334
12 Months or Greater - Fair Value	150,189	379,411
Total - Fair Value	831,021	1,003,745
Less Than 12 Months - Unrealized Losses	(14,720)	(5,426)
12 Months or Greater - Unrealized Losses	(4,607)	(4,476)
Total - Unrealized Losses	(19,327)	(9,902)
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	207,786	57,301
12 Months or Greater - Fair Value	0	5,806
Total - Fair Value	207,786	63,107
Less Than 12 Months - Unrealized Losses	(41)	(200)
12 Months or Greater - Unrealized Losses	0	(10)
Total - Unrealized Losses	(41)	(210)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	37,445	74,859
12 Months or Greater - Fair Value	0	6,957
Total - Fair Value	37,445	81,816
Less Than 12 Months - Unrealized Losses	(1,291)	(847)
12 Months or Greater - Unrealized Losses	0	(10)
Total - Unrealized Losses	(1,291)	(857)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	175,570	221,357
12 Months or Greater - Fair Value	0	43,505
Total - Fair Value	175,570	264,862
Less Than 12 Months - Unrealized Losses	(5,160)	(2,847)
12 Months or Greater - Unrealized Losses	0	(161)
Total - Unrealized Losses	(5,160)	(3,008)
Asset backed securities (“ABS”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	17,415	21,542
12 Months or Greater - Fair Value	0	3,851
Total - Fair Value	17,415	25,393
Less Than 12 Months - Unrealized Losses	(2,716)	(118)
12 Months or Greater - Unrealized Losses	0	(60)
Total - Unrealized Losses	(2,716)	(178)
Residential mortgage backed securities (“RMBS”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	51,169	105,443
12 Months or Greater - Fair Value	4,913	110,452
Total - Fair Value	56,082	215,895
Less Than 12 Months - Unrealized Losses	(284)	(461)
12 Months or Greater - Unrealized Losses	(69)	(2,766)
Total - Unrealized Losses	(353)	(3,227)
Commercial mortgage backed securities (“CMBS”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	28,609	62,388
12 Months or Greater - Fair Value	3,225	11,852
Total - Fair Value	31,834	74,240
Less Than 12 Months - Unrealized Losses	(1,397)	(728)
12 Months or Greater - Unrealized Losses	(14)	(51)
Total - Unrealized Losses	(1,411)	(779)
Collateralized loan obligations (“CLOs”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	162,838	81,444
12 Months or Greater - Fair Value	142,051	196,988
Total - Fair Value	304,889	278,432
Less Than 12 Months - Unrealized Losses	(3,831)	(225)
12 Months or Greater - Unrealized Losses	(4,524)	(1,418)
Total - Unrealized Losses	$ (8,355)	$ (1,643)