Investments - Summary of Fixed Maturities Available for Sale by Component (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 14,188,050	$ 13,251,871
Gross Unrealized Gains	1,273,795	1,069,733
Gross Unrealized Losses	(215,984)	(563,580)
Fair Value	$ 15,245,861	$ 13,758,024
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	$ 381,054	$ 368,718
Gross Unrealized Gains	895	404
Gross Unrealized Losses	(9,151)	(14,078)
Fair Value	$ 372,798	$ 355,044
Percentage of Total Fixed Maturities	3.00%	3.00%
States, municipalities, and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,284,605	$ 1,296,396
Gross Unrealized Gains	126,850	131,516
Gross Unrealized Losses	(1,327)	(1,908)
Fair Value	$ 1,410,128	$ 1,426,004
Percentage of Total Fixed Maturities	9.00%	10.00%
Foreign governments
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 21,701	$ 21,594
Gross Unrealized Gains	1,438	1,369
Gross Unrealized Losses	(62)	(163)
Fair Value	$ 23,077	$ 22,800
Percentage of Total Fixed Maturities	0.00%	0.00%
Corporates
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 11,983,092	$ 11,071,377
Gross Unrealized Gains	1,086,748	872,961
Gross Unrealized Losses	(187,360)	(533,160)
Fair Value	$ 12,882,480	$ 11,411,178
Percentage of Total Fixed Maturities	85.00%	83.00%
Corporates \| Financial
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 2,963,584	$ 2,760,552
Gross Unrealized Gains	285,037	301,624
Gross Unrealized Losses	(45,885)	(54,881)
Fair Value	$ 3,202,736	$ 3,007,295
Percentage of Total Fixed Maturities	21.00%	22.00%
Corporates \| Utilities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,875,946	$ 1,981,241
Gross Unrealized Gains	249,701	223,535
Gross Unrealized Losses	(12,604)	(28,267)
Fair Value	$ 2,113,043	$ 2,176,509
Percentage of Total Fixed Maturities	14.00%	16.00%
Corporates \| Energy
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,542,426	$ 1,568,392
Gross Unrealized Gains	127,989	53,776
Gross Unrealized Losses	(44,324)	(219,101)
Fair Value	$ 1,626,091	$ 1,403,067
Percentage of Total Fixed Maturities	11.00%	10.00%
Corporates \| Other corporate sectors
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 5,601,136	$ 4,761,192
Gross Unrealized Gains	424,021	294,026
Gross Unrealized Losses	(84,547)	(230,911)
Fair Value	$ 5,940,610	$ 4,824,307
Percentage of Total Fixed Maturities	39.00%	35.00%
Collateralized debt obligations
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 60,726	$ 63,662
Gross Unrealized Gains	13,062	16,158
Gross Unrealized Losses	(10,285)	(9,438)
Fair Value	$ 63,503	$ 70,382
Percentage of Total Fixed Maturities	0.00%	1.00%
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 56,410	$ 18,963
Gross Unrealized Gains	621	668
Gross Unrealized Losses	(337)	0
Fair Value	$ 56,694	$ 19,631
Percentage of Total Fixed Maturities	0.00%	0.00%
Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 400,462	$ 411,161
Gross Unrealized Gains	44,181	46,657
Gross Unrealized Losses	(7,462)	(4,833)
Fair Value	$ 437,181	$ 452,985
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	$ 371,862	$ 382,517
Gross Unrealized Gains	43,383	45,926
Gross Unrealized Losses	(7,218)	(4,781)
Fair Value	$ 408,027	$ 423,662
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	$ 28,600	$ 28,644
Gross Unrealized Gains	798	731
Gross Unrealized Losses	(244)	(52)
Fair Value	$ 29,154	$ 29,323
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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