Investments - Summary of Fixed Maturities Available for Sale by Component (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 14,914,580	$ 14,188,050
Fair Value	16,652,913	$ 15,245,861
Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	14,914,580
Gross Unrealized Gains	1,830,971
Gross Unrealized Losses	(92,638)
Fair Value	$ 16,652,913
Percentage of Total Fixed Maturities At Fair Value	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	$ 387,985
Gross Unrealized Gains	12,800
Gross Unrealized Losses	(1,098)
Fair Value	$ 399,687
Percentage of Total Fixed Maturities At Fair Value	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	$ 1,160,537
Gross Unrealized Gains	127,675
Gross Unrealized Losses	(108)
Fair Value	$ 1,288,104
Percentage of Total Fixed Maturities At Fair Value	8.00%
Fixed maturities available for sale [Member] \| Foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 20,939
Gross Unrealized Gains	1,607
Gross Unrealized Losses	0
Fair Value	$ 22,546
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed maturities available for sale [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 12,775,304
Gross Unrealized Gains	1,601,821
Gross Unrealized Losses	(79,275)
Fair Value	$ 14,297,850
Percentage of Total Fixed Maturities At Fair Value	87.00%
Fixed maturities available for sale [Member] \| Corporates [Member] \| Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 3,199,469
Gross Unrealized Gains	426,345
Gross Unrealized Losses	(25,880)
Fair Value	$ 3,599,934
Percentage of Total Fixed Maturities At Fair Value	22.00%
Fixed maturities available for sale [Member] \| Corporates [Member] \| Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,948,519
Gross Unrealized Gains	324,060
Gross Unrealized Losses	(2,655)
Fair Value	$ 2,269,924
Percentage of Total Fixed Maturities At Fair Value	14.00%
Fixed maturities available for sale [Member] \| Corporates [Member] \| Energy [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,602,054
Gross Unrealized Gains	185,296
Gross Unrealized Losses	(28,070)
Fair Value	$ 1,759,280
Percentage of Total Fixed Maturities At Fair Value	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	$ 6,025,262
Gross Unrealized Gains	666,120
Gross Unrealized Losses	(22,670)
Fair Value	$ 6,668,712
Percentage of Total Fixed Maturities At Fair Value	40.00%
Fixed maturities available for sale [Member] \| Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 59,204
Gross Unrealized Gains	19,558
Gross Unrealized Losses	(8,994)
Fair Value	$ 69,768
Percentage of Total Fixed Maturities At Fair Value	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	$ 145,224
Gross Unrealized Gains	5,018
Gross Unrealized Losses	(17)
Fair Value	$ 150,225
Percentage of Total Fixed Maturities At Fair Value	1.00%
Fixed maturities available for sale [Member] \| Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 365,387
Gross Unrealized Gains	62,492
Gross Unrealized Losses	(3,146)
Fair Value	$ 424,733
Percentage of Total Fixed Maturities At Fair Value	2.00%
Fixed maturities available for sale [Member] \| Redeemable preferred stocks [Member] \| Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 336,822
Gross Unrealized Gains	60,159
Gross Unrealized Losses	(3,030)
Fair Value	$ 393,951
Percentage of Total Fixed Maturities At Fair Value	2.00%
Fixed maturities available for sale [Member] \| Redeemable preferred stocks [Member] \| Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 28,565
Gross Unrealized Gains	2,333
Gross Unrealized Losses	(116)
Fair Value	$ 30,782
Percentage of Total Fixed Maturities At Fair Value	0.00%

Percentage of fixed maturities by component at fair value.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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