Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	$ 2,864,747
12 Months or Greater	666,772
Total	3,531,519
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(42,558)
12 Months or Greater	(33,031)
Total	(75,589)
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	77,510
12 Months or Greater	31,491
Total	109,001
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(1,532)
12 Months or Greater	(680)
Total	(2,212)
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	905,755
12 Months or Greater	201,094
Total	1,106,849
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(11,622)
12 Months or Greater	(7,508)
Total	(19,130)
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	1,743,627
12 Months or Greater	148,468
Total	1,892,095
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(26,797)
12 Months or Greater	(7,022)
Total	(33,819)
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	9,423
12 Months or Greater	0
Total	9,423
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(29)
12 Months or Greater	0
Total	(29)
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	14,226
12 Months or Greater	156,842
Total	171,068
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(416)
12 Months or Greater	(10,142)
Total	(10,558)
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	114,206
12 Months or Greater	126,941
Total	241,147
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(2,162)
12 Months or Greater	(7,638)
Total	(9,800)
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	0
12 Months or Greater	1,936
Total	1,936
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	0
12 Months or Greater	(41)
Total	$ (41)
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)