Derivative contracts - gains and losses (Detail) - Finance and Financial Products [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	$ (810)	$ 1,318
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	(810)	1,318
Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	(796)	1,311
Not Designated as Hedging Instrument [Member] | Credit Default [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	$ (14)	$ 7