Borrowings - Gains (Losses) on Debt Repurchases (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure - Borrowings - Gains (Losses) on Debt Repurchases (Detail) [Line Items]
Debt principal repurchased	$ 513	$ 1,467	$ 1,744
Gains (losses) on debt repurchases	$ (3)	$ 1	$ 21

Amount of gain (loss) from the difference between the repurchase price of a debt instrument initially issued by the entity and the net carrying amount of the debt at the time of its repurchase.

No definition available.

Name:	us-gaap_GainLossOnRepurchaseOfDebtInstrument
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration