Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Fixed maturities, fair value of hybrid securities	$ 27	$ 31
Impaired securities	5
Investments securities total unrealized loss in an unrealized loss position for 12 months or more	1	1
Fixed maturity investments, Percentage of amortized costs	70.00%	70.00%
Investments in fair value, deposit with various states	13,686	13,164
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	850	874
Number of securities total unrealized loss in an unrealized loss position for 12 months or more	4	4
Investments securities total unrealized loss in an unrealized loss position for 12 months or more	1	1
Equity Securities
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	1,387	1,014
Investments securities total unrealized loss in an unrealized loss position for 12 months or more	0	0
Procter And Gamble Company, Exxon Mobile Corporation And Chevron Corporation [Member]
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	$ 283
Common Stock
Gain (Loss) on Investments [Line Items]
Number of securities	3