Investments - Summary of Fixed Maturities Available for Sale by Component (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 15,274,490	$ 14,995,101
Fair Value	16,635,885	16,969,325
Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	15,274,490	14,995,101
Gross Unrealized Gains	1,513,436	2,056,907
Gross Unrealized Losses	(152,041)	(82,683)
Fair Value	$ 16,635,885	$ 16,969,325
Percentage of Total Fixed Maturities At Fair Value	100.00%	100.00%
Fixed maturities available for sale [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 394,548	$ 390,646
Gross Unrealized Gains	7,127	18,173
Gross Unrealized Losses	(2,397)	(1,373)
Fair Value	$ 399,278	$ 407,446
Percentage of Total Fixed Maturities At Fair Value	3.00%	2.00%
Fixed maturities available for sale [Member] | States, municipalities, and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,219,749	$ 1,091,960
Gross Unrealized Gains	113,694	127,890
Gross Unrealized Losses	(508)	(135)
Fair Value	$ 1,332,935	$ 1,219,715
Percentage of Total Fixed Maturities At Fair Value	8.00%	7.00%
Fixed maturities available for sale [Member] | Foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 20,541	$ 20,236
Gross Unrealized Gains	1,740	1,782
Gross Unrealized Losses	0	0
Fair Value	$ 22,281	$ 22,018
Percentage of Total Fixed Maturities At Fair Value	0.00%	0.00%
Fixed maturities available for sale [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 13,057,874	$ 12,923,415
Gross Unrealized Gains	1,311,246	1,819,119
Gross Unrealized Losses	(138,203)	(70,698)
Fair Value	$ 14,230,917	$ 14,671,836
Percentage of Total Fixed Maturities At Fair Value	86.00%	87.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 3,329,170	$ 3,282,526
Gross Unrealized Gains	349,415	475,961
Gross Unrealized Losses	(31,568)	(23,392)
Fair Value	$ 3,647,017	$ 3,735,095
Percentage of Total Fixed Maturities At Fair Value	22.00%	22.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,963,247	$ 1,955,737
Gross Unrealized Gains	296,546	369,406
Gross Unrealized Losses	(6,482)	(1,298)
Fair Value	$ 2,253,311	$ 2,323,845
Percentage of Total Fixed Maturities At Fair Value	14.00%	14.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Energy [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,614,917	$ 1,619,349
Gross Unrealized Gains	175,301	226,140
Gross Unrealized Losses	(35,069)	(25,392)
Fair Value	$ 1,755,149	$ 1,820,097
Percentage of Total Fixed Maturities At Fair Value	11.00%	11.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Other corporate sectors [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 6,150,540	$ 6,065,803
Gross Unrealized Gains	489,984	747,612
Gross Unrealized Losses	(65,084)	(20,616)
Fair Value	$ 6,575,440	$ 6,792,799
Percentage of Total Fixed Maturities At Fair Value	39.00%	40.00%
Fixed maturities available for sale [Member] | Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 58,473	$ 59,150
Gross Unrealized Gains	23,518	20,084
Gross Unrealized Losses	(6,467)	(7,653)
Fair Value	$ 75,524	$ 71,581
Percentage of Total Fixed Maturities At Fair Value	0.00%	0.00%
Fixed maturities available for sale [Member] | Other asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 147,511	$ 144,520
Gross Unrealized Gains	3,263	4,835
Gross Unrealized Losses	(362)	0
Fair Value	$ 150,412	$ 149,355
Percentage of Total Fixed Maturities At Fair Value	1.00%	1.00%
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 375,794	$ 365,174
Gross Unrealized Gains	52,848	65,024
Gross Unrealized Losses	(4,104)	(2,824)
Fair Value	$ 424,538	$ 427,374
Percentage of Total Fixed Maturities At Fair Value	2.00%	3.00%
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 347,254	$ 336,621
Gross Unrealized Gains	51,250	62,892
Gross Unrealized Losses	(3,783)	(2,727)
Fair Value	$ 394,721	$ 396,786
Percentage of Total Fixed Maturities At Fair Value	2.00%	3.00%
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member] | Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 28,540	$ 28,553
Gross Unrealized Gains	1,598	2,132
Gross Unrealized Losses	(321)	(97)
Fair Value	$ 29,817	$ 30,588
Percentage of Total Fixed Maturities At Fair Value	0.00%	0.00%