Investments (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013 Security	Sep. 30, 2012	Sep. 30, 2013 Security	Sep. 30, 2012	Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]
Net unrealized appreciation (depreciation) included in OCI	$ (1,000,000)	$ 46,000,000	$ 24,000,000	$ 130,000,000
Net unrealized depreciation included in AOCI	4,000,000		4,000,000		25,000,000
Percentage of mortgage-backed securities represented by investments in US government agency bonds	84.00%		84.00%		85.00%
Credit losses recognized in net income for corporate securities	1,000,000	5,000,000	8,000,000	9,000,000
Credit losses recognized in net income for mortgage-backed securities	0	2,000,000	0	5,000,000
Number of fixed maturities in an unrealized loss position	6,038		6,038
Total number of fixed maturities	24,746		24,746
Largest single unrealized loss in the fixed maturities	3,000,000		3,000,000
Number of equity securities in an unrealized loss position	66		66
Total number of equity securities	183		183
Largest single unrealized loss in the equity securities	48,000,000		48,000,000
Restricted assets in fixed maturities and short-term investments	16,300,000,000		16,300,000,000		16,600,000,000
Restricted assets in cash	$ 129,000,000		$ 129,000,000		$ 139,000,000