Income taxes - Deferred income tax assets and liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Current deferred income tax assets	$ 347us-gaap_DeferredTaxAssetsGrossCurrent	$ 393us-gaap_DeferredTaxAssetsGrossCurrent
Noncurrent deferred income tax assets	172	207
Current deferred income tax liabilities	(4)	(1)
Noncurrent deferred income tax liabilities	(399)	(548)
Net deferred income tax asset	$ 116us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 51us-gaap_DeferredTaxAssetsLiabilitiesNet

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards expected to be realized or consumed within one year or operating cycle, if longer.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 45

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

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.

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-Publisher FASB

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Amount of deferred tax liability attributable to taxable temporary differences expected to be realized or consumed within one year or operating cycle, if longer.

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Gross amount of noncurrent deferred tax liabilities, which result from applying the applicable tax rate to taxable temporary differences pertaining to each jurisdiction to which the entity is obligated to pay income tax.

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