Investments - Summary of Fixed Maturities Available for Sale by Component (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 14,995,101	$ 14,188,050
Gross Unrealized Gains	2,056,907	1,273,795
Gross Unrealized Losses	(82,683)	(215,984)
Fair Value	$ 16,969,325	$ 15,245,861
Percentage of Total Fixed Maturities	100.00%	100.00%
U.S. Government direct, guaranteed, and government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 390,646	$ 381,054
Gross Unrealized Gains	18,173	895
Gross Unrealized Losses	(1,373)	(9,151)
Fair Value	$ 407,446	$ 372,798
Percentage of Total Fixed Maturities	2.00%	3.00%
States, municipalities, and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,091,960	$ 1,284,605
Gross Unrealized Gains	127,890	126,850
Gross Unrealized Losses	(135)	(1,327)
Fair Value	$ 1,219,715	$ 1,410,128
Percentage of Total Fixed Maturities	7.00%	9.00%
Foreign governments
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 20,236	$ 21,701
Gross Unrealized Gains	1,782	1,438
Gross Unrealized Losses	0	(62)
Fair Value	$ 22,018	$ 23,077
Percentage of Total Fixed Maturities	0.00%	0.00%
Corporates
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 12,923,415	$ 11,983,092
Gross Unrealized Gains	1,819,119	1,086,748
Gross Unrealized Losses	(70,698)	(187,360)
Fair Value	$ 14,671,836	$ 12,882,480
Percentage of Total Fixed Maturities	87.00%	85.00%
Corporates \| Financial
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 3,282,526	$ 2,963,584
Gross Unrealized Gains	475,961	285,037
Gross Unrealized Losses	(23,392)	(45,885)
Fair Value	$ 3,735,095	$ 3,202,736
Percentage of Total Fixed Maturities	22.00%	21.00%
Corporates \| Utilities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,955,737	$ 1,875,946
Gross Unrealized Gains	369,406	249,701
Gross Unrealized Losses	(1,298)	(12,604)
Fair Value	$ 2,323,845	$ 2,113,043
Percentage of Total Fixed Maturities	14.00%	14.00%
Corporates \| Energy
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,619,349	$ 1,542,426
Gross Unrealized Gains	226,140	127,989
Gross Unrealized Losses	(25,392)	(44,324)
Fair Value	$ 1,820,097	$ 1,626,091
Percentage of Total Fixed Maturities	11.00%	11.00%
Corporates \| Other corporate sectors
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 6,065,803	$ 5,601,136
Gross Unrealized Gains	747,612	424,021
Gross Unrealized Losses	(20,616)	(84,547)
Fair Value	$ 6,792,799	$ 5,940,610
Percentage of Total Fixed Maturities	40.00%	39.00%
Collateralized debt obligations
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 59,150	$ 60,726
Gross Unrealized Gains	20,084	13,062
Gross Unrealized Losses	(7,653)	(10,285)
Fair Value	$ 71,581	$ 63,503
Percentage of Total Fixed Maturities	0.00%	0.00%
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 144,520	$ 56,410
Gross Unrealized Gains	4,835	621
Gross Unrealized Losses	0	(337)
Fair Value	$ 149,355	$ 56,694
Percentage of Total Fixed Maturities	1.00%	0.00%
Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 365,174	$ 400,462
Gross Unrealized Gains	65,024	44,181
Gross Unrealized Losses	(2,824)	(7,462)
Fair Value	$ 427,374	$ 437,181
Percentage of Total Fixed Maturities	3.00%	3.00%
Redeemable preferred stocks \| Financial
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 336,621	$ 371,862
Gross Unrealized Gains	62,892	43,383
Gross Unrealized Losses	(2,727)	(7,218)
Fair Value	$ 396,786	$ 408,027
Percentage of Total Fixed Maturities	3.00%	3.00%
Redeemable preferred stocks \| Utilities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 28,553	$ 28,600
Gross Unrealized Gains	2,132	798
Gross Unrealized Losses	(97)	(244)
Fair Value	$ 30,588	$ 29,154
Percentage of Total Fixed Maturities	0.00%	0.00%

Percentage of fixed maturities by component at fair value.

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

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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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Amount of debt securities categorized neither as held-to-maturity nor trading.

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