Derivative financial instruments and risk management (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011 Y
Derivative financial instruments and risk management
Management of foreign currency cash flow, maximum period (in years)	P5Y
Deferred net gains, foreign currency exchange rate risk, to be reclassified from equity to current earnings over the next twelve months	$ 1
Deferred net gains (losses), interest rate risk, to be reclassified from equity to current earnings over the next twelve months	$ (4)
Commodity forward and option contracts, maximum period (in years)	5

Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 45

-Subparagraph b(3)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=6887037&loc=d3e80748-113994

Name:	us-gaap_MaximumLengthOfTimeHedgedInForeignCurrencyCashFlowHedge
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration