Derivatives and Hedging Activities - Credit Risk Contingent Feature (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 10
Standard & Poor's, A- Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	1	$ 2
Standard & Poor's, A- Rating [Member] | Two Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	1	2
Standard & Poor's, A- Rating [Member] | Three Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	1	2
Moody's, A3 Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	1	2
Moody's, A3 Rating [Member] | Two Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	1	2
Moody's, A3 Rating [Member] | Three Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 1	$ 2