Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 1,537,660	$ 624,334
12 Months or Greater - Fair Value	183,619	379,411
Total - Fair Value	1,721,279	1,003,745
Less Than 12 Months - Unrealized Losses	(64,016)	(5,426)
12 Months or Greater - Unrealized Losses	(10,070)	(4,476)
Total - Unrealized Losses	(74,086)	(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	12,558	57,301
12 Months or Greater - Fair Value	0	5,806
Total - Fair Value	12,558	63,107
Less Than 12 Months - Unrealized Losses	(21)	(200)
12 Months or Greater - Unrealized Losses	0	(10)
Total - Unrealized Losses	(21)	(210)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	99,354	74,859
12 Months or Greater - Fair Value	2,017	6,957
Total - Fair Value	101,371	81,816
Less Than 12 Months - Unrealized Losses	(3,255)	(847)
12 Months or Greater - Unrealized Losses	(3)	(10)
Total - Unrealized Losses	(3,258)	(857)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	1,034,108	221,357
12 Months or Greater - Fair Value	907	43,505
Total - Fair Value	1,035,015	264,862
Less Than 12 Months - Unrealized Losses	(41,208)	(2,847)
12 Months or Greater - Unrealized Losses	(127)	(161)
Total - Unrealized Losses	(41,335)	(3,008)
Asset backed securities (“ABS”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	64,104	21,542
12 Months or Greater - Fair Value	0	3,851
Total - Fair Value	64,104	25,393
Less Than 12 Months - Unrealized Losses	(6,311)	(118)
12 Months or Greater - Unrealized Losses	0	(60)
Total - Unrealized Losses	(6,311)	(178)
Residential mortgage backed securities (“RMBS”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	7,664	105,443
12 Months or Greater - Fair Value	62,645	110,452
Total - Fair Value	70,309	215,895
Less Than 12 Months - Unrealized Losses	(60)	(461)
12 Months or Greater - Unrealized Losses	(698)	(2,766)
Total - Unrealized Losses	(758)	(3,227)
Commercial mortgage backed securities (“CMBS”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	133,290	62,388
12 Months or Greater - Fair Value	7,593	11,852
Total - Fair Value	140,883	74,240
Less Than 12 Months - Unrealized Losses	(3,086)	(728)
12 Months or Greater - Unrealized Losses	(86)	(51)
Total - Unrealized Losses	(3,172)	(779)
Collateralized loan obligations (“CLOs”)
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	186,582	81,444
12 Months or Greater - Fair Value	110,457	196,988
Total - Fair Value	297,039	278,432
Less Than 12 Months - Unrealized Losses	(10,075)	(225)
12 Months or Greater - Unrealized Losses	(9,156)	(1,418)
Total - Unrealized Losses	$ (19,231)	$ (1,643)