Summary of Fixed Maturities and Equity Securities Available for Sale by Component (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 10,944,104
Gross Unrealized Gains	1,678,815
Gross Unrealized Losses	(102,884)
Fair Value	12,520,035
U.S. Government Direct, Guaranteed, And Government-Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	29,089
Gross Unrealized Gains	1,485
Gross Unrealized Losses	(3)
Fair Value	30,571
% of Total Fixed Maturities	0.00%	[1]
States, Municipalities, And Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,176,752
Gross Unrealized Gains	171,389
Gross Unrealized Losses	0
Fair Value	1,348,141
% of Total Fixed Maturities	11.00%	[1]
Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	22,698
Gross Unrealized Gains	1,118
Gross Unrealized Losses	0
Fair Value	23,816
% of Total Fixed Maturities	0.00%	[1]
Corporates
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	8,784,515
Gross Unrealized Gains	1,462,332
Gross Unrealized Losses	(57,431)
Fair Value	10,189,416
% of Total Fixed Maturities	82.00%	[1]
Collateralized Debt Obligations
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	63,549
Gross Unrealized Gains	0
Gross Unrealized Losses	(25,532)
Fair Value	38,017
% of Total Fixed Maturities	0.00%	[1]
Other Asset-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	43,782
Gross Unrealized Gains	3,144
Gross Unrealized Losses	(704)
Fair Value	46,222
% of Total Fixed Maturities	0.00%	[1]
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	808,844
Gross Unrealized Gains	37,500
Gross Unrealized Losses	(19,151)
Fair Value	827,193
% of Total Fixed Maturities	7.00%	[1]
Total Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	10,929,229
Gross Unrealized Gains	1,676,968
Gross Unrealized Losses	(102,821)
Fair Value	12,503,376
% of Total Fixed Maturities	100.00%	[1]
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	14,875
Gross Unrealized Gains	1,847
Gross Unrealized Losses	(63)
Fair Value	$ 16,659

[1] At fair value