Derivative contracts - gains and losses (Detail) - Finance and Financial Products [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	$ 974	$ 506	$ 2,608
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	974	506	2,608
Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	1,008	108	2,843
Not Designated as Hedging Instrument [Member] | Other, Principally Interest Rate and Foreign Currency [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses		1	(22)
Not Designated as Hedging Instrument [Member] | Credit Default [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	$ (34)	$ 397	$ (213)