Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value
Fair Value, Less than Twelve Months	$ 589,025	$ 4,698,830
Fair Value, Twelve Months or Longer	2,612,614	1,290,990
Fair Value, Total	3,201,639	5,989,820
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(17,565)	(252,192)
Unrealized Loss, Twelve Months or Longer	(187,245)	(176,187)
Unrealized Loss, Total	(204,810)	(428,379)
Investment grade [Member]
Fair Value
Fair Value, Less than Twelve Months	528,412	4,458,883
Fair Value, Twelve Months or Longer	2,339,468	1,091,128
Fair Value, Total	2,867,880	5,550,011
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(14,337)	(229,105)
Unrealized Loss, Twelve Months or Longer	(112,237)	(106,067)
Unrealized Loss, Total	(126,574)	(335,172)
Investment grade [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Fair Value
Fair Value, Less than Twelve Months	6	37,182
Fair Value, Twelve Months or Longer	61,709	89,664
Fair Value, Total	61,715	126,846
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	0	(212)
Unrealized Loss, Twelve Months or Longer	(1,034)	(2,575)
Unrealized Loss, Total	(1,034)	(2,787)
Investment grade [Member] | States, municipalities, and political subdivisions [Member]
Fair Value
Fair Value, Less than Twelve Months	14,974	124,907
Fair Value, Twelve Months or Longer	1,056	7,981
Fair Value, Total	16,030	132,888
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(26)	(1,648)
Unrealized Loss, Twelve Months or Longer	(5)	(102)
Unrealized Loss, Total	(31)	(1,750)
Investment grade [Member] | Corporates [Member]
Fair Value
Fair Value, Less than Twelve Months	513,334	4,252,191
Fair Value, Twelve Months or Longer	2,272,240	993,483
Fair Value, Total	2,785,574	5,245,674
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(14,311)	(226,611)
Unrealized Loss, Twelve Months or Longer	(111,175)	(103,390)
Unrealized Loss, Total	(125,486)	(330,001)
Investment grade [Member] | Corporates [Member] | Financial [Member]
Fair Value
Fair Value, Less than Twelve Months	69,791	931,161
Fair Value, Twelve Months or Longer	388,946	241,442
Fair Value, Total	458,737	1,172,603
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(1,004)	(36,337)
Unrealized Loss, Twelve Months or Longer	(15,440)	(21,572)
Unrealized Loss, Total	(16,444)	(57,909)
Investment grade [Member] | Corporates [Member] | Utilities [Member]
Fair Value
Fair Value, Less than Twelve Months	2,023	329,753
Fair Value, Twelve Months or Longer	177,061	121,308
Fair Value, Total	179,084	451,061
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(2)	(11,680)
Unrealized Loss, Twelve Months or Longer	(5,553)	(9,442)
Unrealized Loss, Total	(5,555)	(21,122)
Investment grade [Member] | Corporates [Member] | Energy [Member]
Fair Value
Fair Value, Less than Twelve Months	48,948	475,736
Fair Value, Twelve Months or Longer	145,645	54,937
Fair Value, Total	194,593	530,673
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(693)	(29,426)
Unrealized Loss, Twelve Months or Longer	(7,662)	(9,382)
Unrealized Loss, Total	(8,355)	(38,808)
Investment grade [Member] | Corporates [Member] | Other corporate sectors [Member]
Fair Value
Fair Value, Less than Twelve Months	392,572	2,515,541
Fair Value, Twelve Months or Longer	1,560,588	575,796
Fair Value, Total	1,953,160	3,091,337
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(12,612)	(149,168)
Unrealized Loss, Twelve Months or Longer	(82,520)	(62,994)
Unrealized Loss, Total	(95,132)	(212,162)
Investment grade [Member] | Other asset-backed securities [Member]
Fair Value
Fair Value, Less than Twelve Months	98	44,603
Fair Value, Twelve Months or Longer	4,463	0
Fair Value, Total	4,561	44,603
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	0	(634)
Unrealized Loss, Twelve Months or Longer	(23)	0
Unrealized Loss, Total	(23)	(634)
Below investment grade [Member]
Fair Value
Fair Value, Less than Twelve Months	60,613	239,947
Fair Value, Twelve Months or Longer	273,146	199,862
Fair Value, Total	333,759	439,809
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(3,228)	(23,087)
Unrealized Loss, Twelve Months or Longer	(75,008)	(70,120)
Unrealized Loss, Total	(78,236)	(93,207)
Below investment grade [Member] | States, municipalities, and political subdivisions [Member]
Fair Value
Fair Value, Less than Twelve Months	85	0
Fair Value, Twelve Months or Longer	0	0
Fair Value, Total	85	0
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(143)	0
Unrealized Loss, Twelve Months or Longer	0	0
Unrealized Loss, Total	(143)	0
Below investment grade [Member] | Corporates [Member]
Fair Value
Fair Value, Less than Twelve Months	46,396	239,947
Fair Value, Twelve Months or Longer	259,034	186,276
Fair Value, Total	305,430	426,223
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(2,742)	(23,087)
Unrealized Loss, Twelve Months or Longer	(69,120)	(63,706)
Unrealized Loss, Total	(71,862)	(86,793)
Below investment grade [Member] | Corporates [Member] | Financial [Member]
Fair Value
Fair Value, Less than Twelve Months	0	22,087
Fair Value, Twelve Months or Longer	104,759	81,101
Fair Value, Total	104,759	103,188
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	0	(8,674)
Unrealized Loss, Twelve Months or Longer	(28,021)	(20,932)
Unrealized Loss, Total	(28,021)	(29,606)
Below investment grade [Member] | Corporates [Member] | Utilities [Member]
Fair Value
Fair Value, Less than Twelve Months	0	28,613
Fair Value, Twelve Months or Longer	12,648	0
Fair Value, Total	12,648	28,613
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	0	(3,277)
Unrealized Loss, Twelve Months or Longer	(4,135)	0
Unrealized Loss, Total	(4,135)	(3,277)
Below investment grade [Member] | Corporates [Member] | Energy [Member]
Fair Value
Fair Value, Less than Twelve Months	11,207	42,874
Fair Value, Twelve Months or Longer	47,260	36,122
Fair Value, Total	58,467	78,996
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(589)	(3,901)
Unrealized Loss, Twelve Months or Longer	(18,465)	(19,662)
Unrealized Loss, Total	(19,054)	(23,563)
Below investment grade [Member] | Corporates [Member] | Other corporate sectors [Member]
Fair Value
Fair Value, Less than Twelve Months	35,189	146,373
Fair Value, Twelve Months or Longer	94,367	69,053
Fair Value, Total	129,556	215,426
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(2,153)	(7,235)
Unrealized Loss, Twelve Months or Longer	(18,499)	(23,112)
Unrealized Loss, Total	(20,652)	(30,347)
Below investment grade [Member] | Collateralized debt obligations [Member]
Fair Value
Fair Value, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	14,112	13,586
Fair Value, Total	14,112	13,586
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	0	0
Unrealized Loss, Twelve Months or Longer	(5,888)	(6,414)
Unrealized Loss, Total	(5,888)	(6,414)
Below investment grade [Member] | Other asset-backed securities [Member]
Fair Value
Fair Value, Less than Twelve Months	14,132	0
Fair Value, Twelve Months or Longer	0	0
Fair Value, Total	14,132	0
Unrealized Loss
Unrealized Loss, Less Than Twelve Months	(343)	0
Unrealized Loss, Twelve Months or Longer	0	0
Unrealized Loss, Total	$ (343)	$ 0