Derivative financial instruments - Summary of information about the fair values of derivative instruments on consolidated balance sheets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative Assets	$ 13	$ 336
Derivative Liabilities	(93)	(37)
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	3	245
Derivative Liabilities	(92)	(1)
Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	2	225
Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(89)	(1)
Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Other Long-term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	1	20
Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Other Long-term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(3)	0
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	10	91
Derivative Liabilities	(1)	(36)
Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	10	81
Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ (1)	(34)
Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Other Long-term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets		10
Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Other Long-term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities		$ (2)