Invested Assets - Components of Unrealized Appreciation or Depreciation of Investments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Fixed maturities, Gross unrealized appreciation	$ 2,522	$ 2,704
Fixed maturities, Gross unrealized depreciation	40	26
Fixed maturities, Unrealized appreciation (depreciation), Net	2,482	2,678
Equity securities, Gross Unrealized Appreciation	635	453
Equity securities, Gross Unrealized Depreciation	14	34
Equity securities, Unrealized appreciation (depreciation), Net	621	419
Total fixed maturities and equity securities, Unrealized appreciation (depreciation), Net	3,103	3,097
Deferred income tax liability	1,086	1,084
Total fixed maturities and equity securities, Unrealized appreciation (depreciation), Net of tax	$ 2,017	$ 2,013