Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012 Investment	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Investment grade Commercial Mortgage Backed Securities	$ 27
Unrealized investment gains due to a net gain position	1,923	1,542
Fixed maturities, fair value of hybrid securities	31
Other-than-temporary impairment charges	0	0
Impaired securities	7
Investments securities total unrealized loss in an unrealized loss position for 12 months or more	2	5
Procter And Gamble Company And Exxon Mobile Corporation
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	300
Two Primary Contributors
Gain (Loss) on Investments [Line Items]
Number of securities	3
Debt Securities
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	887	713
Number of securities total unrealized loss in an unrealized loss position for 12 months or more	8	20
Investments securities total unrealized loss in an unrealized loss position for 12 months or more	2	5
Debt Securities | Minimum
Gain (Loss) on Investments [Line Items]
Percentage of investment amortized cost	70.00%	70.00%
Equity Securities
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	1,036	829
Number of securities total unrealized loss in an unrealized loss position for 12 months or more	0	2
Equity Securities | Minimum
Gain (Loss) on Investments [Line Items]
Percentage of investment amortized cost		70.00%
Equity Securities | Maximum
Gain (Loss) on Investments [Line Items]
Investments securities total unrealized loss in an unrealized loss position for 12 months or more		$ 1