Investments - Summary of Fixed Maturities and Equity Securities Available for Sale by Component (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 12,962,830	$ 12,823,612
Debt Securities Fair Value	14,894,514	14,493,060
Equity Securities Cost or Amortized Cost	776	776
Equity Securities Fair Value	1,474	1,477
Total Available for Sale Securities Cost	12,963,606
Gross Unrealized Gains	1,998,002
Gross Unrealized Losses	(65,620)
Total Available for Sale Securities Fair Value	14,895,988us-gaap_AvailableForSaleSecurities
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities Cost or Amortized Cost	776
Gross Unrealized Gains	698
Gross Unrealized Losses	0
Equity Securities Fair Value	1,474
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	12,962,830
Gross Unrealized Gains	1,997,304
Gross Unrealized Losses	(65,620)
Debt Securities Fair Value	14,894,514
Percentage of Total Fixed Maturities At Fair Value	100.00%
Fixed Maturities [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	369,684
Gross Unrealized Gains	14,107
Gross Unrealized Losses	(791)
Debt Securities Fair Value	383,000
Percentage of Total Fixed Maturities At Fair Value	3.00%
Fixed Maturities [Member] \| States, Municipalities, and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	1,281,007
Gross Unrealized Gains	183,084
Gross Unrealized Losses	(423)
Debt Securities Fair Value	1,463,668
Percentage of Total Fixed Maturities At Fair Value	10.00%
Fixed Maturities [Member] \| Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	27,372
Gross Unrealized Gains	1,918
Gross Unrealized Losses	(1)
Debt Securities Fair Value	29,289
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed Maturities [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	10,704,410
Gross Unrealized Gains	1,714,355
Gross Unrealized Losses	(51,682)
Debt Securities Fair Value	12,367,083
Percentage of Total Fixed Maturities At Fair Value	83.00%
Fixed Maturities [Member] \| Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	65,642
Gross Unrealized Gains	16,784
Gross Unrealized Losses	(7,872)
Debt Securities Fair Value	74,554
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed Maturities [Member] \| Other Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	20,384
Gross Unrealized Gains	1,555
Gross Unrealized Losses	0
Debt Securities Fair Value	21,939
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed Maturities [Member] \| Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	494,331
Gross Unrealized Gains	65,501
Gross Unrealized Losses	(4,851)
Debt Securities Fair Value	554,981
Percentage of Total Fixed Maturities At Fair Value	4.00%
Financial [Member] \| Fixed Maturities [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	2,609,600
Gross Unrealized Gains	471,670
Gross Unrealized Losses	(13,593)
Debt Securities Fair Value	3,067,677
Percentage of Total Fixed Maturities At Fair Value	20.00%
Financial [Member] \| Fixed Maturities [Member] \| Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	465,655
Gross Unrealized Gains	64,554
Gross Unrealized Losses	(4,828)
Debt Securities Fair Value	525,381
Percentage of Total Fixed Maturities At Fair Value	4.00%
Utilities [Member] \| Fixed Maturities [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	2,147,011
Gross Unrealized Gains	434,773
Gross Unrealized Losses	(524)
Debt Securities Fair Value	2,581,260
Percentage of Total Fixed Maturities At Fair Value	18.00%
Utilities [Member] \| Fixed Maturities [Member] \| Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	28,676
Gross Unrealized Gains	947
Gross Unrealized Losses	(23)
Debt Securities Fair Value	29,600
Percentage of Total Fixed Maturities At Fair Value	0.00%
Energy [Member] \| Fixed Maturities [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	1,546,541
Gross Unrealized Gains	197,320
Gross Unrealized Losses	(24,426)
Debt Securities Fair Value	1,719,435
Percentage of Total Fixed Maturities At Fair Value	12.00%
Other Corporate Sectors [Member] \| Fixed Maturities [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	4,401,258
Gross Unrealized Gains	610,592
Gross Unrealized Losses	(13,139)
Debt Securities Fair Value	$ 4,998,711
Percentage of Total Fixed Maturities At Fair Value	33.00%

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

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