Derivative contracts - gains and losses (Detail) - Finance and Financial Products [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	$ 718	$ 751	$ 974
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	718	751	974
Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	$ 718	662	1,008
Not Designated as Hedging Instrument [Member] | Other [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses		$ 89	$ (34)