Investments - Summary of Fixed Maturities and Equity Securities Available for Sale by Component (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 13,489,492	$ 13,251,871
Debt Securities Fair Value	14,459,605	13,758,024
Equity Securities Cost or Amortized Cost	776	776
Equity Securities Fair Value	1,610	$ 1,635
Total Available for Sale Securities Cost	13,490,268
Gross Unrealized Gains	1,364,811
Gross Unrealized Losses	(393,864)
Total Available for Sale Securities Fair Value	14,461,215
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities Cost or Amortized Cost	776
Gross Unrealized Gains	834
Gross Unrealized Losses	0
Equity Securities Fair Value	1,610
Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	13,489,492
Gross Unrealized Gains	1,363,977
Gross Unrealized Losses	(393,864)
Debt Securities Fair Value	$ 14,459,605
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	$ 371,811
Gross Unrealized Gains	13,985
Gross Unrealized Losses	(1,422)
Debt Securities Fair Value	$ 384,374
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	$ 1,281,548
Gross Unrealized Gains	156,149
Gross Unrealized Losses	(318)
Debt Securities Fair Value	$ 1,437,379
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	$ 21,338
Gross Unrealized Gains	2,037
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 23,375
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	$ 11,322,964
Gross Unrealized Gains	1,126,378
Gross Unrealized Losses	(376,914)
Debt Securities Fair Value	$ 12,072,428
Percentage of Total Fixed Maturities At Fair Value	84.00%
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	$ 62,370
Gross Unrealized Gains	14,169
Gross Unrealized Losses	(11,047)
Debt Securities Fair Value	$ 65,492
Percentage of Total Fixed Maturities At Fair Value	0.00%
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	$ 18,471
Gross Unrealized Gains	793
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 19,264
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	$ 410,990
Gross Unrealized Gains	50,466
Gross Unrealized Losses	(4,163)
Debt Securities Fair Value	$ 457,293
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	$ 2,731,785
Gross Unrealized Gains	301,713
Gross Unrealized Losses	(60,705)
Debt Securities Fair Value	$ 2,972,793
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	$ 382,357
Gross Unrealized Gains	48,911
Gross Unrealized Losses	(4,163)
Debt Securities Fair Value	$ 427,105
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	$ 1,981,725
Gross Unrealized Gains	292,523
Gross Unrealized Losses	(20,505)
Debt Securities Fair Value	$ 2,253,743
Percentage of Total Fixed Maturities At Fair Value	16.00%
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	$ 28,633
Gross Unrealized Gains	1,555
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 30,188
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	$ 1,569,187
Gross Unrealized Gains	54,215
Gross Unrealized Losses	(182,124)
Debt Securities Fair Value	$ 1,441,278
Percentage of Total Fixed Maturities At Fair Value	10.00%
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	$ 5,040,267
Gross Unrealized Gains	477,927
Gross Unrealized Losses	(113,580)
Debt Securities Fair Value	$ 5,404,614
Percentage of Total Fixed Maturities At Fair Value	37.00%

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