Income taxes - Uncertain tax positions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, January 1	$ 91us-gaap_UnrecognizedTaxBenefits	$ 184us-gaap_UnrecognizedTaxBenefits	$ 210us-gaap_UnrecognizedTaxBenefits
Additions based on tax positions related to the current year	10	7	12
Additions for tax positions of prior years	52	19	45
Reductions for tax positions of prior years	(9)	(10)	(92)
Settlements with tax authorities	(36)	(96)	39
Expiration of the statute of limitations for assessing taxes	0	(13)	(30)
Balance, December 31	108us-gaap_UnrecognizedTaxBenefits	91us-gaap_UnrecognizedTaxBenefits	184us-gaap_UnrecognizedTaxBenefits
Interest income (expense) recognized in the year ended December 31	6	(10)	32
Interest receivable (payable) as of December 31	$ 0	$ (5)	$ 8

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Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of interest expense accrued for an underpayment of income taxes.

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Amount of interest expense for an underpayment of income taxes.

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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