Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months	$ 513,007	$ 2,026,193
12 Months or Greater	1,499,842	1,554,704
Total	2,012,849	3,580,897
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months	(4,748)	(28,520)
12 Months or Greater	(23,068)	(49,384)
Total	(27,816)	(77,904)
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months	0	23,710
12 Months or Greater	68,951	69,146
Total	68,951	92,856
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months	0	(15)
12 Months or Greater	(589)	(1,061)
Total	(589)	(1,076)
Obligations of U.S. states and political subdivisions
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months	17,518	316,655
12 Months or Greater	213,567	358,086
Total	231,085	674,741
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months	(444)	(3,875)
12 Months or Greater	(1,782)	(7,110)
Total	(2,226)	(10,985)
Corporate debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months	149,625	1,272,279
12 Months or Greater	852,102	785,627
Total	1,001,727	2,057,906
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months	(1,315)	(18,130)
12 Months or Greater	(8,333)	(22,384)
Total	(9,648)	(40,514)
Asset backed securities (“ABS”)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months	18,444	51,324
12 Months or Greater	0	0
Total	18,444	51,324
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months	(33)	(146)
12 Months or Greater	0	0
Total	(33)	(146)
Residential mortgage backed securities (“RMBS”)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months	0	24
12 Months or Greater	175,437	178,573
Total	175,437	178,597
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months	0	0
12 Months or Greater	(7,859)	(10,309)
Total	(7,859)	(10,309)
Commercial mortgage backed securities (“CMBS”)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months	9,757	65,704
12 Months or Greater	189,785	163,272
Total	199,542	228,976
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months	(94)	(1,060)
12 Months or Greater	(4,505)	(8,520)
Total	(4,599)	(9,580)
Collateralized loan obligations (“CLO”)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months	317,663	296,497
12 Months or Greater	0	0
Total	317,663	296,497
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months	(2,862)	(5,294)
12 Months or Greater	0	0
Total	$ (2,862)	$ (5,294)