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Financial instruments - Forward exchange contracts (Details) - Forward exchange contract € in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2017 EUR (€) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 EUR (€)
Disclosure of financial assets [line items] |  |  |  |
Hedge ineffectiveness | $ 0 |  |  |
Cash flow hedges |  |  |  |
Disclosure of financial assets [line items] |  |  |  |
Notional amount | € |  | € 109 |  | € 92
Negative fair value | $ 800,000 |  |  |
Positive fair value |  |  | $ 300,000 |

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