Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 333,520	$ 2,026,193
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	683,194	1,554,704
Total	1,016,714	3,580,897
Less Than 12 Months	(4,460)	(28,520)
12 Months or Greater	(6,940)	(49,384)
Total	(11,400)	(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	12,483	23,710
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	45,423	69,146
Total	57,906	92,856
Less Than 12 Months	(19)	(15)
12 Months or Greater	(108)	(1,061)
Total	(127)	(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	2,283	316,655
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	57,193	358,086
Total	59,476	674,741
Less Than 12 Months	(468)	(3,875)
12 Months or Greater	(180)	(7,110)
Total	(648)	(10,985)
Corporate debt securities
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	29,333	1,272,279
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	298,419	785,627
Total	327,752	2,057,906
Less Than 12 Months	(1,987)	(18,130)
12 Months or Greater	(1,147)	(22,384)
Total	(3,134)	(40,514)
Asset backed securities (“ABS”)
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	9,257	51,324
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Total	9,257	51,324
Less Than 12 Months	(102)	(146)
12 Months or Greater	0	0
Total	(102)	(146)
Residential mortgage backed securities (“RMBS”)
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	32,181	24
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	158,325	178,573
Total	190,506	178,597
Less Than 12 Months	(297)	0
12 Months or Greater	(4,413)	(10,309)
Total	(4,710)	(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	0	65,704
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	78,697	163,272
Total	78,697	228,976
Less Than 12 Months	0	(1,060)
12 Months or Greater	(750)	(8,520)
Total	(750)	(9,580)
Collateralized loan obligations (“CLO”)
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	247,983	296,497
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	45,137	0
Total	293,120	296,497
Less Than 12 Months	(1,587)	(5,294)
12 Months or Greater	(342)	0
Total	$ (1,929)	$ (5,294)