Investments - Summary of Fixed Maturities and Equity Securities Available for Sale by Component (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	$ 12,267,490	$ 11,963,406	[1]
Total fixed maturities available for sale, Fair Value	12,756,917	13,541,193	[1]
Equity Securities Cost or Amortized Cost	875	14,875	[1]
Total Available for Sale Securities Cost	12,268,365
Gross Unrealized Gains	865,977
Gross Unrealized Losses	(375,771)
Total Available for Sale Securities Fair Value	12,758,571
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities Cost or Amortized Cost	875
Gross Unrealized Gains	779
Gross Unrealized Losses	0
Equity Securities Fair Value	1,654
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	12,267,490
Gross Unrealized Gains	865,198
Gross Unrealized Losses	(375,771)
Total fixed maturities available for sale, Fair Value	12,756,917
% 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]
Debt Securities Cost or Amortized Cost	449,177
Gross Unrealized Gains	414
Gross Unrealized Losses	(55,301)
Total fixed maturities available for sale, Fair Value	394,290
% 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]
Debt Securities Cost or Amortized Cost	1,277,916
Gross Unrealized Gains	84,528
Gross Unrealized Losses	(10,428)
Total fixed maturities available for sale, Fair Value	1,352,016
% of Total Fixed Maturities At Fair Value	11.00%
Fixed Maturities [Member] | Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	31,161
Gross Unrealized Gains	521
Gross Unrealized Losses	(46)
Total fixed maturities available for sale, Fair Value	31,636
% of Total Fixed Maturities At Fair Value	0.00%
Fixed Maturities [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	9,882,319
Gross Unrealized Gains	748,720
Gross Unrealized Losses	(278,103)
Total fixed maturities available for sale, Fair Value	10,352,936
% of Total Fixed Maturities At Fair Value	81.00%
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	67,950
Gross Unrealized Gains	0
Gross Unrealized Losses	(14,877)
Total fixed maturities available for sale, Fair Value	53,073
% of Total Fixed Maturities At Fair Value	1.00%
Fixed Maturities [Member] | Other Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	36,113
Gross Unrealized Gains	2,592
Gross Unrealized Losses	(277)
Total fixed maturities available for sale, Fair Value	38,428
% of Total Fixed Maturities At Fair Value	0.00%
Fixed Maturities [Member] | Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	522,854
Gross Unrealized Gains	28,423
Gross Unrealized Losses	(16,739)
Total fixed maturities available for sale, Fair Value	$ 534,538
% of Total Fixed Maturities At Fair Value	4.00%

[1] Derived from audited financial statements