Derivative instruments (Details Textual) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Derivative Instruments Gain Loss [Line Items]
Length of time hedged in foreign currency contracts			3 years
Interest expense, net [Member]
Derivative Instruments Gain Loss [Line Items]
Unrealized gain (loss) on the hedged debt	$ 83	$ (63)	$ (6)	$ (125)
Unrealized gain (loss) on interest rate swaps	(83)	$ 63	6	$ 125
Foreign currency forward contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Notional amount of terminated derivatives	1,700
Foreign currency forward contracts [Member] | Net cash provided by operating activities [Member]
Derivative Instruments Gain Loss [Line Items]
Proceeds from counterparties on termination of derivatives	247
Foreign currency forward contracts [Member] | Derivatives designated as hedging instrument [Member]
Derivative Instruments Gain Loss [Line Items]
Derivative, notional amount	3,200		3,200		$ 3,800
Foreign currency forward contracts [Member] | Derivatives not designated as hedging instrument [Member]
Derivative Instruments Gain Loss [Line Items]
Derivative, notional amount	928		928		875
Foreign currency option contracts [Member] | Derivatives designated as hedging instrument [Member]
Derivative Instruments Gain Loss [Line Items]
Derivative, notional amount	382		382		271
Foreign currency and cross currency swap contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Amounts expected to be reclassified from AOCI into earnings over the next 12 months	427		427
Forward interest rate contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Amounts expected to be reclassified from AOCI into earnings over the next 12 months	(1)		(1)
Interest rate swap [Member] | Derivatives designated as hedging instrument [Member]
Derivative Instruments Gain Loss [Line Items]
Derivative, notional amount	$ 6,650		$ 6,650		$ 6,650
Rate adjustment to LIBOR on interest rate swap agreements [Member] | LIBOR [Member]
Derivative Instruments Gain Loss [Line Items]
Derivative lower range variable interest rate	0.40%		0.40%
Derivative higher range variable interest rate	2.00%		2.00%