Derivative contracts - gains and losses (Detail) (Finance and Financial Products [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	$ 506	$ 2,608	$ 1,963
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	506	2,608	1,963
Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	108	2,843	997
Not Designated as Hedging Instrument [Member] | Credit Default Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains/losses	397	(213)	894
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)	$ 72