Investments - Summary of Fixed Maturities Available for Sale by Component (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 14,602,250	$ 14,188,050
Fair Value	15,874,131	$ 15,245,861
Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	14,602,250
Gross Unrealized Gains	1,431,648
Gross Unrealized Losses	(159,767)
Fair Value	$ 15,874,131
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	$ 385,755
Gross Unrealized Gains	1,830
Gross Unrealized Losses	(6,424)
Fair Value	$ 381,161
Percentage of Total Fixed Maturities At Fair Value	3.00%
Fixed maturities available for sale [Member] \| States, municipalities, and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,184,062
Gross Unrealized Gains	120,066
Gross Unrealized Losses	(1,029)
Fair Value	$ 1,303,099
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	$ 20,066
Gross Unrealized Gains	1,466
Gross Unrealized Losses	(8)
Fair Value	$ 21,524
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	$ 12,447,636
Gross Unrealized Gains	1,239,746
Gross Unrealized Losses	(135,040)
Fair Value	$ 13,552,342
Percentage of Total Fixed Maturities At Fair Value	85.00%
Fixed maturities available for sale [Member] \| Corporates [Member] \| Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 3,137,495
Gross Unrealized Gains	331,704
Gross Unrealized Losses	(34,260)
Fair Value	$ 3,434,939
Percentage of Total Fixed Maturities At Fair Value	21.00%
Fixed maturities available for sale [Member] \| Corporates [Member] \| Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,896,115
Gross Unrealized Gains	268,397
Gross Unrealized Losses	(9,842)
Fair Value	$ 2,154,670
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	$ 1,589,982
Gross Unrealized Gains	145,310
Gross Unrealized Losses	(31,320)
Fair Value	$ 1,703,972
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	$ 5,824,044
Gross Unrealized Gains	494,335
Gross Unrealized Losses	(59,618)
Fair Value	$ 6,258,761
Percentage of Total Fixed Maturities At Fair Value	39.00%
Fixed maturities available for sale [Member] \| Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 59,976
Gross Unrealized Gains	17,515
Gross Unrealized Losses	(9,648)
Fair Value	$ 67,843
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	$ 104,476
Gross Unrealized Gains	1,072
Gross Unrealized Losses	(361)
Fair Value	$ 105,187
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	$ 400,279
Gross Unrealized Gains	49,953
Gross Unrealized Losses	(7,257)
Fair Value	$ 442,975
Percentage of Total Fixed Maturities At Fair Value	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	$ 371,691
Gross Unrealized Gains	48,325
Gross Unrealized Losses	(7,011)
Fair Value	$ 413,005
Percentage of Total Fixed Maturities At Fair Value	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	$ 28,588
Gross Unrealized Gains	1,628
Gross Unrealized Losses	(246)
Fair Value	$ 29,970
Percentage of Total Fixed Maturities At Fair Value	0.00%

Percentage of fixed maturities by component at fair value.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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