Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 391,108	$ 2,026,193
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	339,648	1,554,704
Total	730,756	3,580,897
Less Than 12 Months	(4,911)	(28,520)
12 Months or Greater	(3,475)	(49,384)
Total	(8,386)	(77,904)
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	25,284	23,710
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	7,364	69,146
Total	32,648	92,856
Less Than 12 Months	(48)	(15)
12 Months or Greater	(27)	(1,061)
Total	(75)	(1,076)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	4,965	316,655
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	10,007	358,086
Total	14,972	674,741
Less Than 12 Months	(467)	(3,875)
12 Months or Greater	(27)	(7,110)
Total	(494)	(10,985)
Corporate debt securities
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	135,541	1,272,279
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	78,842	785,627
Total	214,383	2,057,906
Less Than 12 Months	(2,818)	(18,130)
12 Months or Greater	(333)	(22,384)
Total	(3,151)	(40,514)
Asset backed securities (“ABS”)
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	8,998	51,324
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Total	8,998	51,324
Less Than 12 Months	(65)	(146)
12 Months or Greater	0	0
Total	(65)	(146)
Residential mortgage backed securities (“RMBS”)
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	47,166	24
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	116,534	178,573
Total	163,700	178,597
Less Than 12 Months	(309)	0
12 Months or Greater	(2,354)	(10,309)
Total	(2,663)	(10,309)
Commercial mortgage backed securities (“CMBS”)
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	21,882	65,704
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	13,579	163,272
Total	35,461	228,976
Less Than 12 Months	(196)	(1,060)
12 Months or Greater	(70)	(8,520)
Total	(266)	(9,580)
Collateralized loan obligations (“CLOs”)
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	147,272	296,497
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	113,322	0
Total	260,594	296,497
Less Than 12 Months	(1,008)	(5,294)
12 Months or Greater	(664)	0
Total	$ (1,672)	$ (5,294)