Investments - Summary of Fixed Maturities and Equity Securities Available for Sale by Component (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 13,166,248	$ 12,823,612
Debt Securities Fair Value	14,080,199	14,493,060
Equity Securities Cost or Amortized Cost	776	776
Equity Securities Fair Value	1,582	$ 1,477
Total Available for Sale Securities Cost	13,167,024
Gross Unrealized Gains	1,276,967
Gross Unrealized Losses	(362,210)
Total Available for Sale Securities Fair Value	14,081,781
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities Cost or Amortized Cost	776
Gross Unrealized Gains	806
Gross Unrealized Losses	0
Equity Securities Fair Value	1,582
Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	13,166,248
Gross Unrealized Gains	1,276,161
Gross Unrealized Losses	(362,210)
Debt Securities Fair Value	$ 14,080,199
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]
Fixed Maturities, Available for Sale, Amortized Cost	$ 373,835
Gross Unrealized Gains	1,618
Gross Unrealized Losses	(5,696)
Debt Securities Fair Value	$ 369,757
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]
Fixed Maturities, Available for Sale, Amortized Cost	$ 1,296,976
Gross Unrealized Gains	138,075
Gross Unrealized Losses	(1,374)
Debt Securities Fair Value	$ 1,433,677
Percentage of Total Fixed Maturities At Fair Value	10.00%
Fixed maturities available for sale [Member] \| Foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 20,905
Gross Unrealized Gains	1,545
Gross Unrealized Losses	(4)
Debt Securities Fair Value	$ 22,446
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]
Fixed Maturities, Available for Sale, Amortized Cost	$ 10,979,970
Gross Unrealized Gains	1,067,354
Gross Unrealized Losses	(342,112)
Debt Securities Fair Value	$ 11,705,212
Percentage of Total Fixed Maturities At Fair Value	83.00%
Fixed maturities available for sale [Member] \| Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 63,960
Gross Unrealized Gains	16,745
Gross Unrealized Losses	(8,593)
Debt Securities Fair Value	$ 72,112
Percentage of Total Fixed Maturities At Fair Value	1.00%
Fixed maturities available for sale [Member] \| Other asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 19,273
Gross Unrealized Gains	978
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 20,251
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed maturities available for sale [Member] \| Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 411,329
Gross Unrealized Gains	49,846
Gross Unrealized Losses	(4,431)
Debt Securities Fair Value	$ 456,744
Percentage of Total Fixed Maturities At Fair Value	3.00%
Financial [Member] \| Fixed maturities available for sale [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 2,688,462
Gross Unrealized Gains	340,450
Gross Unrealized Losses	(44,834)
Debt Securities Fair Value	$ 2,984,078
Percentage of Total Fixed Maturities At Fair Value	21.00%
Financial [Member] \| Fixed maturities available for sale [Member] \| Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 382,674
Gross Unrealized Gains	48,618
Gross Unrealized Losses	(4,431)
Debt Securities Fair Value	$ 426,861
Percentage of Total Fixed Maturities At Fair Value	3.00%
Utilities [Member] \| Fixed maturities available for sale [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 2,105,221
Gross Unrealized Gains	274,626
Gross Unrealized Losses	(21,681)
Debt Securities Fair Value	$ 2,358,166
Percentage of Total Fixed Maturities At Fair Value	17.00%
Utilities [Member] \| Fixed maturities available for sale [Member] \| Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 28,655
Gross Unrealized Gains	1,228
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 29,883
Percentage of Total Fixed Maturities At Fair Value	0.00%
Energy [Member] \| Fixed maturities available for sale [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 1,538,147
Gross Unrealized Gains	89,072
Gross Unrealized Losses	(111,771)
Debt Securities Fair Value	$ 1,515,448
Percentage of Total Fixed Maturities At Fair Value	11.00%
Other corporate sectors [Member] \| Fixed maturities available for sale [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 4,648,140
Gross Unrealized Gains	363,206
Gross Unrealized Losses	(163,826)
Debt Securities Fair Value	$ 4,847,520
Percentage of Total Fixed Maturities At Fair Value	34.00%

Percentage of fixed maturities by component at fair value.

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