Derivative financial instruments - Carrying and notional amounts of hedges (Details) - Cash flow hedges $ in Millions	Dec. 31, 2020 USD ($) MMBTU	Dec. 31, 2019 USD ($)
Disclosure of detailed information about financial instruments [line items]
Hedging instrument, assets	$ 4	$ 1
Hedging instrument, liabilities	0	(4)
Highly probable forecast capital expenditure \| Foreign exchange risk
Disclosure of detailed information about financial instruments [line items]
Hedging instrument, assets	4	1
Hedging instrument, liabilities	0	(4)
Nominal amounts of hedging instruments	162	$ 150
Highly probable forecast sales \| Commodity price risk
Disclosure of detailed information about financial instruments [line items]
Hedging instrument, assets	0
Hedging instrument, liabilities	$ 0
Nominal Amounts Of Hedging Instruments \| MMBTU	(175)
Highly probable forecast sales \| Commodity price risk \| Within one year
Disclosure of detailed information about financial instruments [line items]
Nominal Amounts Of Hedging Instruments \| MMBTU	135
Highly probable forecast sales \| Commodity price risk \| 1 to 2 years
Disclosure of detailed information about financial instruments [line items]
Nominal Amounts Of Hedging Instruments \| MMBTU	40

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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