Derivatives and Hedging Activities - Credit Risk Contingent Feature (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Credit Derivatives [Line Items]
One rating downgrade	$ 204
Moody's, A3 Rating | One Rating Downgrade
Credit Derivatives [Line Items]
One rating downgrade	6	11	11
Moody's, A3 Rating | Two Rating Downgrade
Credit Derivatives [Line Items]
One rating downgrade	11	16	16
Moody's, A3 Rating | Three Rating Downgrade
Credit Derivatives [Line Items]
One rating downgrade	13	16	16
Standard & Poor's, A- Rating | One Rating Downgrade
Credit Derivatives [Line Items]
One rating downgrade	6	11	11
Standard & Poor's, A- Rating | Two Rating Downgrade
Credit Derivatives [Line Items]
One rating downgrade	11	16	16
Standard & Poor's, A- Rating | Three Rating Downgrade
Credit Derivatives [Line Items]
One rating downgrade	$ 13	$ 16	$ 16