Income taxes - Uncertain tax positions (Details) - USD ($) $ in Millions |
12 Months Ended | ||
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Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
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| Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] | |||
| Balance, January 1 | $ 108 | $ 91 | $ 184 |
| Additions based on tax positions related to the current year | 11 | 10 | 7 |
| Additions for tax positions of prior years | 3 | 52 | 19 |
| Reductions for tax positions of prior years | (21) | (9) | (10) |
| Settlements with tax authorities | (17) | (36) | (96) |
| Expiration of the statute of limitations for assessing taxes | 0 | 0 | (13) |
| Balance, December 31 | 84 | 108 | 91 |
| Interest income (expense) recognized in the year ended December 31 | 8 | 6 | (10) |
| Interest receivable (payable) as of December 31 | $ 9 | $ 0 | $ (5) |
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. No definition available.
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- Definition Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Amount of interest expense accrued for an underpayment of income taxes. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Amount of interest expense for an underpayment of income taxes. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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