Invested Assets - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012 Investment	Dec. 31, 2011
Schedule of Available-For-Sale Securities [Line Items]
Fixed maturities other-than-temporary impairment losses in gross unrealized depreciation		$ 3
Approximate number of individual fixed maturities in unrealized loss position	180
Approximate number of individual equity securities in unrealized loss position	30
Cumulative credit losses recognized in net income	$ 22	$ 20