Derivatives and Hedging Activities - Credit Risk Contingent Feature (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 169
Moody's, A3 Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	4	1	6
Moody's, A3 Rating [Member] | Two Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	4	1	11
Moody's, A3 Rating [Member] | Three Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	6	3	11
Standard & Poor's, A- Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	4	1	6
Standard & Poor's, A- Rating [Member] | Two Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	4	1	11
Standard & Poor's, A- Rating [Member] | Three Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 6	$ 3	$ 11