Derivative contracts - gains and losses (Detail) - Finance and Financial Products [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	$ (65)	$ 20	$ 395	$ (790)
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	(65)	20	395	(790)
Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	$ (65)	(83)	$ 395	(879)
Not Designated as Hedging Instrument [Member] | Credit Default [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses		$ 103		$ 89