Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Fair Value, Less than Twelve Months	$ 4,831,670	$ 498,363
Fair Value, Twelve Months or Longer	711,560	605,288
Fair Value, Total	5,543,230	1,103,651
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(210,438)	(9,332)
Unrealized Loss, Twelve Months or Longer	(103,138)	(73,351)
Unrealized Loss, Total	(313,576)	(82,683)
Investment grade [Member]
Fair Value
Fair Value, Less than Twelve Months	4,731,873	464,715
Fair Value, Twelve Months or Longer	473,170	354,546
Fair Value, Total	5,205,043	819,261
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(202,249)	(4,057)
Unrealized Loss, Twelve Months or Longer	(45,134)	(11,154)
Unrealized Loss, Total	(247,383)	(15,211)
Investment grade [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Fair Value
Fair Value, Less than Twelve Months	291,797	34,388
Fair Value, Twelve Months or Longer	66,160	47,514
Fair Value, Total	357,957	81,902
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(8,844)	(422)
Unrealized Loss, Twelve Months or Longer	(2,799)	(951)
Unrealized Loss, Total	(11,643)	(1,373)
Investment grade [Member] | States, municipalities, and political subdivisions [Member]
Fair Value
Fair Value, Less than Twelve Months	181,894	4,561
Fair Value, Twelve Months or Longer	1,632	1,771
Fair Value, Total	183,526	6,332
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(4,832)	(21)
Unrealized Loss, Twelve Months or Longer	(21)	(9)
Unrealized Loss, Total	(4,853)	(30)
Investment grade [Member] | Corporates [Member]
Fair Value
Fair Value, Less than Twelve Months	4,183,169	425,766
Fair Value, Twelve Months or Longer	399,784	299,308
Fair Value, Total	4,582,953	725,074
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(186,976)	(3,614)
Unrealized Loss, Twelve Months or Longer	(41,900)	(10,097)
Unrealized Loss, Total	(228,876)	(13,711)
Investment grade [Member] | Corporates [Member] | Financial [Member]
Fair Value
Fair Value, Less than Twelve Months	934,194	133,080
Fair Value, Twelve Months or Longer	69,993	35,302
Fair Value, Total	1,004,187	168,382
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(38,148)	(652)
Unrealized Loss, Twelve Months or Longer	(5,518)	(1,429)
Unrealized Loss, Total	(43,666)	(2,081)
Investment grade [Member] | Corporates [Member] | Utilities [Member]
Fair Value
Fair Value, Less than Twelve Months	420,921	48,562
Fair Value, Twelve Months or Longer	45,084	32,345
Fair Value, Total	466,005	80,907
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(17,417)	(569)
Unrealized Loss, Twelve Months or Longer	(3,540)	(729)
Unrealized Loss, Total	(20,957)	(1,298)
Investment grade [Member] | Corporates [Member] | Energy [Member]
Fair Value
Fair Value, Less than Twelve Months	216,903	23,463
Fair Value, Twelve Months or Longer	49,052	67,775
Fair Value, Total	265,955	91,238
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(8,981)	(81)
Unrealized Loss, Twelve Months or Longer	(7,689)	(3,682)
Unrealized Loss, Total	(16,670)	(3,763)
Investment grade [Member] | Corporates [Member] | Other corporate sectors [Member]
Fair Value
Fair Value, Less than Twelve Months	2,611,151	220,661
Fair Value, Twelve Months or Longer	235,655	163,886
Fair Value, Total	2,846,806	384,547
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(122,430)	(2,312)
Unrealized Loss, Twelve Months or Longer	(25,153)	(4,257)
Unrealized Loss, Total	(147,583)	(6,569)
Investment grade [Member] | Other asset-backed securities [Member]
Fair Value
Fair Value, Less than Twelve Months	64,963
Fair Value, Twelve Months or Longer	0
Fair Value, Total	64,963
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(872)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss, Total	(872)
Investment grade [Member] | Redeemable preferred stocks [Member]
Fair Value
Fair Value, Less than Twelve Months	10,050	0
Fair Value, Twelve Months or Longer	5,594	5,953
Fair Value, Total	15,644	5,953
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(725)	0
Unrealized Loss, Twelve Months or Longer	(414)	(97)
Unrealized Loss, Total	(1,139)	(97)
Investment grade [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Fair Value
Fair Value, Less than Twelve Months	10,050
Fair Value, Twelve Months or Longer	0
Fair Value, Total	10,050
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(725)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss, Total	(725)
Investment grade [Member] | Redeemable preferred stocks [Member] | Utilities [Member]
Fair Value
Fair Value, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	5,594	5,953
Fair Value, Total	5,594	5,953
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	0	0
Unrealized Loss, Twelve Months or Longer	(414)	(97)
Unrealized Loss, Total	(414)	(97)
Below investment grade [Member]
Fair Value
Fair Value, Less than Twelve Months	99,797	33,648
Fair Value, Twelve Months or Longer	238,390	250,742
Fair Value, Total	338,187	284,390
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(8,189)	(5,275)
Unrealized Loss, Twelve Months or Longer	(58,004)	(62,197)
Unrealized Loss, Total	(66,193)	(67,472)
Below investment grade [Member] | States, municipalities, and political subdivisions [Member]
Fair Value
Fair Value, Less than Twelve Months		200
Fair Value, Twelve Months or Longer		0
Fair Value, Total		200
Unrealized Loss
Unrealized Loss, Less Than Twelve Months		(105)
Unrealized Loss, Twelve Months or Longer		0
Unrealized Loss, Total		(105)
Below investment grade [Member] | Corporates [Member]
Fair Value
Fair Value, Less than Twelve Months	99,797	33,448
Fair Value, Twelve Months or Longer	199,776	214,019
Fair Value, Total	299,573	247,467
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(8,189)	(5,170)
Unrealized Loss, Twelve Months or Longer	(49,535)	(51,817)
Unrealized Loss, Total	(57,724)	(56,987)
Below investment grade [Member] | Corporates [Member] | Financial [Member]
Fair Value
Fair Value, Less than Twelve Months	24,873	0
Fair Value, Twelve Months or Longer	62,073	84,432
Fair Value, Total	86,946	84,432
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(5,895)	0
Unrealized Loss, Twelve Months or Longer	(12,884)	(21,311)
Unrealized Loss, Total	(18,779)	(21,311)
Below investment grade [Member] | Corporates [Member] | Utilities [Member]
Fair Value
Fair Value, Less than Twelve Months	15,059
Fair Value, Twelve Months or Longer	0
Fair Value, Total	15,059
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(48)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss, Total	(48)
Below investment grade [Member] | Corporates [Member] | Energy [Member]
Fair Value
Fair Value, Less than Twelve Months	12,647	8,114
Fair Value, Twelve Months or Longer	68,890	75,204
Fair Value, Total	81,537	83,318
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(533)	(104)
Unrealized Loss, Twelve Months or Longer	(20,775)	(21,525)
Unrealized Loss, Total	(21,308)	(21,629)
Below investment grade [Member] | Corporates [Member] | Other corporate sectors [Member]
Fair Value
Fair Value, Less than Twelve Months	47,218	25,334
Fair Value, Twelve Months or Longer	68,813	54,383
Fair Value, Total	116,031	79,717
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(1,713)	(5,066)
Unrealized Loss, Twelve Months or Longer	(15,876)	(8,981)
Unrealized Loss, Total	(17,589)	(14,047)
Below investment grade [Member] | Collateralized debt obligations [Member]
Fair Value
Fair Value, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	15,003	12,347
Fair Value, Total	15,003	12,347
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	0	0
Unrealized Loss, Twelve Months or Longer	(4,997)	(7,653)
Unrealized Loss, Total	(4,997)	(7,653)
Below investment grade [Member] | Redeemable preferred stocks [Member]
Fair Value
Fair Value, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	23,611	24,376
Fair Value, Total	23,611	24,376
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	0	0
Unrealized Loss, Twelve Months or Longer	(3,472)	(2,727)
Unrealized Loss, Total	(3,472)	(2,727)
Below investment grade [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Fair Value
Fair Value, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	23,611	24,376
Fair Value, Total	23,611	24,376
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	0	0
Unrealized Loss, Twelve Months or Longer	(3,472)	(2,727)
Unrealized Loss, Total	$ (3,472)	$ (2,727)