Investments (Narrative) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013 Security	Mar. 31, 2012	Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]
Net unrealized appreciation (depreciation) included in OCI	$ 24,000,000	$ 68,000,000
Net unrealized depreciation included in AOCI	0		(25,000,000)
Percentage of mortgage-backed securities represented by investments in US government agency bonds	85.00%		85.00%
Credit losses recognized in net income for corporate securities	1,000,000	3,000,000
Credit losses recognized in net income for mortgage-backed securities	0	3,000,000
Number of fixed maturities in an unrealized loss position	2,073
Total number of fixed maturities	23,742
Largest single unrealized loss in the fixed maturities	(2,000,000)
Number of equity securities in an unrealized loss position	57
Total number of equity securities	189
Largest single unrealized loss in the equity securities	(1,000,000)
Restricted assets in fixed maturities and short-term investments	16,500,000,000		16,600,000,000
Restricted assets in cash	$ 153,000,000		$ 139,000,000