Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less Than 12 Months	$ 2,875,083
12 Months or Greater	933,768
Total	3,808,851
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(45,073)
12 Months or Greater	(46,765)
Total	(91,838)
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	89,759
12 Months or Greater	78,769
Total	168,528
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(583)
12 Months or Greater	(2,770)
Total	(3,353)
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	643,147
12 Months or Greater	245,763
Total	888,910
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(12,356)
12 Months or Greater	(9,252)
Total	(21,608)
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	1,782,640
12 Months or Greater	299,549
Total	2,082,189
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(29,184)
12 Months or Greater	(13,121)
Total	(42,305)
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	70,935
12 Months or Greater	0
Total	70,935
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(546)
12 Months or Greater	0
Total	(546)
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	4,930
12 Months or Greater	149,648
Total	154,578
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(183)
12 Months or Greater	(11,411)
Total	(11,594)
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	81,972
12 Months or Greater	160,039
Total	242,011
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(1,763)
12 Months or Greater	(10,211)
Total	(11,974)
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	201,700
12 Months or Greater	0
Total	201,700
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less Than 12 Months	(458)
12 Months or Greater	0
Total	$ (458)
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)