Investments (Narrative) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 partnerships Security	Dec. 31, 2011	Dec. 31, 2010
Investments, Debt and Equity Securities [Abstract]
Net unrealized appreciation (depreciation) included in OCI	$ 137,000,000	$ (48,000,000)
Net unrealized depreciation included in AOCI	(25,000,000)	(155,000,000)
Percentage of mortgage-backed securities represented by investments in US government agency bonds	85.00%	84.00%
Portion of gross unrealized loss represented by the United States Treasury and Agency obligations	18,000,000
Company assumed recovery rate	32.00%
Moodys historical mean recovery rate	42.00%
Credit losses recognized in net income for corporate securities	14,000,000	9,000,000	14,000,000
Credit losses recognized in net income for mortgage-backed securities	6,000,000	11,000,000	32,000,000
Limited partnerships number	65
Trading securities - mutual funds	212,000,000	162,000,000
Trading securities - equity securities	23,000,000	24,000,000
Trading securities - fixed maturities	8,000,000	8,000,000
Number of fixed maturities in an unrealized loss position	2,029
Total number of fixed maturities	23,679
Largest single unrealized loss in the fixed maturities	5,000,000
Number of equity securities in an unrealized loss position	56
Total number of equity securities	193
Largest single unrealized loss in the equity securities	1,000,000
Restricted assets in fixed maturities and short-term investments	16,600,000,000	14,900,000,000
Restricted assets in cash	$ 139,000,000	$ 179,000,000