Derivative contracts (Detail) - Finance and Financial Products [Member] - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Liabilities		$ 2,187	$ 2,890
Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative [Line Items]
Liabilities		2,187	2,890
Notional Value	[1]	$ 28,494	$ 26,497

[1] Represents the aggregate undiscounted amounts payable assuming that the value of each index is zero at each contract's expiration date. Certain of these contracts are denominated in foreign currencies. Notional amounts are based on the foreign currency exchange rates as of each balance sheet date.