Summary of Fixed Maturities and Equity Securities Available for Sale by Component (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	$ 12,210,939		$ 11,963,406	[1]
Total fixed maturities available for sale, Fair Value	12,862,567		13,541,193	[1]
Equity Securities Cost or Amortized Cost	875		14,875	[1]
Total Available for Sale Securities Cost	12,211,814
Gross Unrealized Gains	973,968
Gross Unrealized Losses	(321,628)
Total Available for Sale Securities Fair Value	12,864,154
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Equity Securities Cost or Amortized Cost	875
Gross Unrealized Gains	712
Gross Unrealized Losses	0
Equity Securities Fair Value	1,587
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	12,210,939
Gross Unrealized Gains	973,256
Gross Unrealized Losses	(321,628)
Total fixed maturities available for sale, Fair Value	12,862,567
% of Total Fixed Maturities	100.00%	[2]
Fixed Maturities | U.S. Government Direct, Guaranteed, And Government-Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	446,825
Gross Unrealized Gains	481
Gross Unrealized Losses	(42,735)
Total fixed maturities available for sale, Fair Value	404,571
% of Total Fixed Maturities	3.00%	[2]
Fixed Maturities | States, Municipalities, And Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	1,272,586
Gross Unrealized Gains	113,140
Gross Unrealized Losses	(6,505)
Total fixed maturities available for sale, Fair Value	1,379,221
% of Total Fixed Maturities	11.00%	[2]
Fixed Maturities | Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	32,123
Gross Unrealized Gains	588
Gross Unrealized Losses	(76)
Total fixed maturities available for sale, Fair Value	32,635
% of Total Fixed Maturities	0.00%	[2]
Fixed Maturities | Corporates
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	9,786,976
Gross Unrealized Gains	820,146
Gross Unrealized Losses	(242,876)
Total fixed maturities available for sale, Fair Value	10,364,246
% of Total Fixed Maturities	81.00%	[2]
Fixed Maturities | Collateralized Debt Obligations
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	66,822
Gross Unrealized Gains	0
Gross Unrealized Losses	(14,605)
Total fixed maturities available for sale, Fair Value	52,217
% of Total Fixed Maturities	0.00%	[2]
Fixed Maturities | Other Asset-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	37,108
Gross Unrealized Gains	2,741
Gross Unrealized Losses	(145)
Total fixed maturities available for sale, Fair Value	39,704
% of Total Fixed Maturities	0.00%	[2]
Fixed Maturities | Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Debt Securities Cost or Amortized Cost	568,499
Gross Unrealized Gains	36,160
Gross Unrealized Losses	(14,686)
Total fixed maturities available for sale, Fair Value	$ 589,973
% of Total Fixed Maturities	5.00%	[2]

[1] Derived from audited financial statements

[2] At fair value