Derivatives and Hedging Activities - Credit Risk Contingent Feature (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 181
Moody's, A3 Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	1	6
Moody's, A3 Rating [Member] | Two Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	1	11
Moody's, A3 Rating [Member] | Three Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	3	11
Standard & Poor's, A- Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	1	6
Standard & Poor's, A- Rating [Member] | Two Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	1	11
Standard & Poor's, A- Rating [Member] | Three Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 3	$ 11