Investments - Summary of Fixed Maturities and Equity Securities Available for Sale by Component (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 13,126,918	$ 12,823,612
Debt Securities Fair Value	14,121,462	14,493,060
Equity Securities Cost or Amortized Cost	776	776
Equity Securities Fair Value	1,545	$ 1,477
Total Available for Sale Securities Cost	13,127,694
Gross Unrealized Gains	1,251,617
Gross Unrealized Losses	(256,304)
Total Available for Sale Securities Fair Value	14,123,007
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities Cost or Amortized Cost	776
Gross Unrealized Gains	769
Gross Unrealized Losses	0
Equity Securities Fair Value	1,545
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	13,126,918
Gross Unrealized Gains	1,250,848
Gross Unrealized Losses	(256,304)
Debt Securities Fair Value	$ 14,121,462
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	$ 371,529
Gross Unrealized Gains	449
Gross Unrealized Losses	(17,422)
Debt Securities Fair Value	$ 354,556
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	$ 1,298,664
Gross Unrealized Gains	132,850
Gross Unrealized Losses	(2,151)
Debt Securities Fair Value	$ 1,429,363
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	$ 23,037
Gross Unrealized Gains	1,425
Gross Unrealized Losses	(379)
Debt Securities Fair Value	$ 24,083
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	$ 10,870,294
Gross Unrealized Gains	1,049,334
Gross Unrealized Losses	(221,937)
Debt Securities Fair Value	$ 11,697,691
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	$ 64,782
Gross Unrealized Gains	16,525
Gross Unrealized Losses	(7,639)
Debt Securities Fair Value	$ 73,668
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	$ 19,450
Gross Unrealized Gains	1,182
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 20,632
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	$ 479,162
Gross Unrealized Gains	49,083
Gross Unrealized Losses	(6,776)
Debt Securities Fair Value	$ 521,469
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	$ 2,675,863
Gross Unrealized Gains	314,572
Gross Unrealized Losses	(38,366)
Debt Securities Fair Value	$ 2,952,069
Percentage of Total Fixed Maturities At Fair Value	21.00%
Financial [Member] \| Fixed Maturities [Member] \| Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 450,496
Gross Unrealized Gains	48,332
Gross Unrealized Losses	(6,754)
Debt Securities Fair Value	$ 492,074
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	$ 2,138,355
Gross Unrealized Gains	258,616
Gross Unrealized Losses	(27,006)
Debt Securities Fair Value	$ 2,369,965
Percentage of Total Fixed Maturities At Fair Value	17.00%
Utilities [Member] \| Fixed Maturities [Member] \| Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 28,666
Gross Unrealized Gains	751
Gross Unrealized Losses	(22)
Debt Securities Fair Value	$ 29,395
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	$ 1,541,450
Gross Unrealized Gains	115,442
Gross Unrealized Losses	(46,261)
Debt Securities Fair Value	$ 1,610,631
Percentage of Total Fixed Maturities At Fair Value	11.00%
Other Corporate Sectors [Member] \| Fixed Maturities [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 4,514,626
Gross Unrealized Gains	360,704
Gross Unrealized Losses	(110,304)
Debt Securities Fair Value	$ 4,765,026
Percentage of Total Fixed Maturities At Fair Value	34.00%

Percentage of fixed maturities by component at fair value.

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