Assets and Liabilities Measured at Fair Value on a Recurring Basis (Parenthetical) (Detail) (Equity Securities, Other [Member], Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Equity Securities, Other [Member] | Fair Value, Measurements, Recurring [Member]
Federal Home Loan Bank (FHLB) of Atlanta stock stated at par value	$ 432	$ 342
Federal Reserve Bank stock stated at par value	398	398
Mutual fund investments (par value)	$ 202	$ 187