Invested Assets - Components of Unrealized Appreciation or Depreciation of Investments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-For-Sale Securities [Line Items]
Fixed maturities, Gross unrealized appreciation	$ 2,859	$ 2,495
Fixed maturities, Gross unrealized depreciation	26	73
Fixed maturities, Unrealized appreciation (depreciation), Net	2,833	2,422
Equity securities, Gross unrealized appreciation	413	319
Equity securities, Gross unrealized depreciation	34	71
Equity securities, Unrealized appreciation (depreciation), Net	379	248
Total fixed maturities and equity securities, Unrealized appreciation (depreciation), Net	3,212	2,670
Deferred income tax liability	1,124	934
Total fixed maturities and equity securities, Unrealized appreciation (depreciation), Net of tax	$ 2,088	$ 1,736