Fair Value Measurements - Additional Information (Detail) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Interest Rates	Dec. 31, 2011 Credit Spreads	Dec. 31, 2011 Interest Rate Volatility	Dec. 31, 2011 Fair Value Based on Discounted Cash Flow Pricing Model	Dec. 31, 2011 Fair Value, Measurements, Recurring	Dec. 31, 2010 Fair Value, Measurements, Recurring	Dec. 31, 2011 Fair Value, Measurements, Recurring Level 2	Dec. 31, 2009 Fair Value, Measurements, Recurring Level 2
Fair Value, Measurement Inputs, Disclosure [Line Items]
Percentage of fair value of investments valued by obtaining quoted market prices from investment custodian									49.00%
Percentage of holdings valued by obtaining quoted market prices from investment custodian									77.00%
Percentage of holdings of investments using discounted cash flow pricing model									11.00%
Percentage of fair value of investments using DCF pricing model									45.00%
Percentage of fair value of investments using other pricing services									6.00%
Percentage of holdings of investments using other pricing services									12.00%
Reverse dual currency investments fair value							$ 2,300,000,000	$ 2,700,000,000
Amount of Level 2 investments that were reclassified upon the adoption of new accounting guidance										1,000,000,000
Magnitude of movement			0.50%	0.50%	50.00%
Fair values of available-for-sale fixed maturity and perpetual securities valued by our DCF pricing model						19,700,000,000
Securities held to maturity, fixed maturities, fair value	$ 46,383,000,000	$ 30,469,000,000				$ 25,800,000,000