Derivative instruments (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Interest expense, net [Member]	Jun. 30, 2012 Interest expense, net [Member]	Jun. 30, 2013 Interest expense, net [Member]	Jun. 30, 2012 Interest expense, net [Member]	Jun. 30, 2013 Foreign currency and cross currency swap contracts [Member]	Jun. 30, 2013 Forward interest rate contracts [Member]	Jun. 30, 2013 Rate adjustment to LIBOR on interest rate swap agreements [Member]	Jun. 30, 2012 Rate adjustment to LIBOR on interest rate swap agreements [Member]	Jun. 30, 2013 Designated as Hedging Instrument [Member] Interest rate swap [Member]	Mar. 31, 2013 Designated as Hedging Instrument [Member] Interest rate swap [Member]	Jun. 30, 2012 Designated as Hedging Instrument [Member] Interest rate swap [Member]	Jun. 30, 2013 Not Designated as Hedging Instrument [Member] Foreign currency forward contracts [Member]	Dec. 31, 2012 Not Designated as Hedging Instrument [Member] Foreign currency forward contracts [Member]	Jun. 30, 2013 Designated as Hedging Instrument [Member] Foreign currency forward contracts [Member]	Dec. 31, 2012 Designated as Hedging Instrument [Member] Foreign currency forward contracts [Member]	Jun. 30, 2013 Designated as Hedging Instrument [Member] Foreign currency option contracts [Member]	Dec. 31, 2012 Designated as Hedging Instrument [Member] Foreign currency option contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Length of time hedged in foreign currency contracts			3 years
Derivative, notional amount													$ 1,900	$ 2,500	$ 3,600	$ 652	$ 629	$ 3,800	$ 3,700	$ 123	$ 200
Ineffective portions of cash flow hedging instruments (approximately), gain (loss)	0	1	1	0
Amounts expected to be reclassified from Accumulated Other Comprehensive Income into earnings over the next 12 months - gains (losses)									41
Amounts expected to be reclassified from Accumulated Other Comprehensive Income into earnings over the next 12 months - gain (loss)										(1)
Derivative lower range variable interest rate											1.10%	0.30%
Derivative higher range variable interest rate											2.00%	2.60%
Unrealized gain (loss) on the hedged debt					113	(38)	91	(20)
Unrealized gain (loss) on interest rate swaps					$ (113)	$ 38	$ (91)	$ 20