Invested Assets - Change in Unrealized Appreciation or Depreciation of Investments Carried at Fair Value (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Investments Debt And Equity Securities [Abstract]
Change in unrealized appreciation of fixed maturities	$ (196)	$ (17)
Change in unrealized appreciation of equity securities	202	102
Change in unrealized appreciation of fixed maturities and equity securities	6	85
Deferred income tax	2	30
Total change in unrealized appreciation (depreciation) of fixed maturities and equity securities, Net of tax	$ 4	$ 55