Derivatives and Hedging Activities - Largest Exposure to Individual Counterparty (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Derivative [Line Items]
Net exposure after netting adjustments and collateral	$ 793	$ 609	$ 413
Largest Individual Counterparty [Member]
Derivative [Line Items]
Largest gross exposure (derivative asset) to an individual counterparty	137	133	106
Collateral posted by this counterparty	55	100	44
Derivative liability with this counterparty	78	31	103
Collateral pledged to this counterparty			47
Net exposure after netting adjustments and collateral	$ 4	$ 2	$ 6