Investments (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012 Security	Sep. 30, 2011	Sep. 30, 2012 Security	Sep. 30, 2011	Dec. 31, 2011
Investment [Line Items]
Net unrealized appreciation (depreciation) included in OCI	$ 46,000,000	$ 30,000,000	$ 130,000,000	$ 38,000,000
Net unrealized depreciation included in AOCI	28,000,000		28,000,000		155,000,000
Percentage of mortgage-backed securities represented by investments in US government agency bonds	86.00%		86.00%		84.00%
Credit losses recognized in net income for corporate securities	5,000,000	4,000,000	9,000,000	4,000,000
Credit losses recognized in net income for mortgage-backed securities	2,000,000	5,000,000	5,000,000	8,000,000
Number of fixed maturities in an unrealized loss position	1,473		1,473
Total number of fixed maturities	23,322		23,322
Largest single unrealized loss in the fixed maturities	4,000,000		4,000,000
Number of equity securities in an unrealized loss position	68		68
Total number of equity securities	197		197
Largest single unrealized loss in the equity securities	1,000,000		1,000,000
Restricted assets in fixed maturities and short-term investments	16,300,000,000		16,300,000,000		14,900,000,000
Restricted assets in cash	$ 116,000,000		$ 116,000,000		$ 179,000,000