Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	$ 2,908,284
12 Months or Greater	708,095
Total	3,616,379
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(49,868)
12 Months or Greater	(35,179)
Total	(85,047)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months		$ 1,667,170
12 Months or Greater		752,693
Total		2,419,863
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months		(9,357)
12 Months or Greater		(22,206)
Total		(31,563)
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	85,817
12 Months or Greater	34,027
Total	119,844
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(1,826)
12 Months or Greater	(862)
Total	(2,688)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months		144,042
12 Months or Greater		31,196
Total		175,238
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months		(796)
12 Months or Greater		(482)
Total		(1,278)
Obligations of U.S. states and political subdivisions
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	778,354
12 Months or Greater	207,827
Total	986,181
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(9,776)
12 Months or Greater	(6,910)
Total	(16,686)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months		505,311
12 Months or Greater		211,684
Total		716,995
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months		(3,624)
12 Months or Greater		(5,125)
Total		(8,749)
Corporate debt securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	1,787,815
12 Months or Greater	176,254
Total	1,964,069
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(34,534)
12 Months or Greater	(8,225)
Total	(42,759)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months		932,350
12 Months or Greater		200,716
Total		1,133,066
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months		(4,288)
12 Months or Greater		(4,881)
Total		(9,169)
Asset backed securities (“ABS”)
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	71,293
12 Months or Greater	0
Total	71,293
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(333)
12 Months or Greater	0
Total	(333)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months		4,923
12 Months or Greater		0
Total		4,923
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months		(2)
12 Months or Greater		0
Total		(2)
Residential mortgage backed securities (“RMBS”)
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	5,099
12 Months or Greater	158,215
Total	163,314
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(163)
12 Months or Greater	(10,407)
Total	(10,570)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months		14,979
12 Months or Greater		166,329
Total		181,308
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months		(280)
12 Months or Greater		(7,084)
Total		(7,364)
Commercial mortgage backed securities (“CMBS”)
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	121,944
12 Months or Greater	130,590
Total	252,534
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(3,098)
12 Months or Greater	(8,749)
Total	(11,847)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months		51,096
12 Months or Greater		138,769
Total		189,865
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months		(358)
12 Months or Greater		(4,548)
Total		(4,906)
Collateralized loan obligations (“CLO”)
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	57,962
12 Months or Greater	1,182
Total	59,144
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(138)
12 Months or Greater	(26)
Total	$ (164)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months		14,243
12 Months or Greater		3,568
Total		17,811
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months		(7)
12 Months or Greater		(72)
Total		(79)
Equity securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months		226
12 Months or Greater		431
Total		657
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract]
Less Than 12 Months		(2)
12 Months or Greater		(14)
Total		$ (16)