Derivatives and Hedging Activities - Credit Risk Contingent Feature (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 43
Moody's, A3 Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	2	$ 2
Moody's, A3 Rating [Member] | Two Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	2	2
Moody's, A3 Rating [Member] | Three Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	2	4
Standard & Poor's, A- Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	2	2
Standard & Poor's, A- Rating [Member] | Two Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	2	2
Standard & Poor's, A- Rating [Member] | Three Rating Downgrades [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 2	$ 4