Schedule of Positions in Derivative Instruments (Parenthetical) (Detail) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Derivative [Line Items]
Reduction in derivative assets	$ 473
Reduction in derivative liabilities	$ 206

The increase (decrease) during the period in the carrying value of derivative instruments reported as liabilities that are due to be disposed of within one year (or the normal operating cycle, if longer).

No definition available.

Name:	us-gaap_IncreaseDecreaseInDerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration