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Financial instruments - Forward exchange contracts (Details) - Forward exchange contract € in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 EUR (€)
Disclosure of financial assets [line items]
Hedge ineffectiveness	$ 0
Cash flow hedges
Disclosure of financial assets [line items]
Notional amount | €		€ 18		€ 45
Positive fair value			$ 300,000
Negative fair value	$ 400,000

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The fair value of financial assets. [Refer: At fair value [member]; Financial assets]

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- Definition

The fair value of financial liabilities. [Refer: At fair value [member]; Financial liabilities]

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The gain (loss) on hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item.

+ References

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The nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

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