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Financial instruments - Natural gas forward contracts (Details) MMBTU in Thousands | 12 Months Ended
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Dec. 31, 2021 USD ($) MMBTU | Dec. 31, 2020 USD ($) MMBTU
Disclosure of financial assets [line items] |  |
Reclassification of hedge losses (gains) in cash flow hedge | $ (12,654,000) | $ 20,271,000
Accumulated other comprehensive income |  |
Disclosure of financial assets [line items] |  |
Reclassification of hedge losses (gains) in cash flow hedge | (12,654,000) | $ 20,271,000
Natural gas forward contract |  |
Disclosure of financial assets [line items] |  |
Hedge ineffectiveness | $ 0 |
Natural gas forward contract | Cash flow hedges |  |
Disclosure of financial assets [line items] |  |
Hedging instrument, energy (in mmbtu) | MMBTU | 322,880 | 306,820
Notional amount | $ 1,053,917,000 | $ 1,005,550,000
Liabilities, fair value | (3,986,000) | (177,426,000)
Natural gas forward contract | Cash flow hedges | other current assets |  |
Disclosure of financial assets [line items] |  |
Assets, fair value | 5,905,000 | 410,000
Natural gas forward contract | Cash flow hedges | Other non-current asset |  |
Disclosure of financial assets [line items] |  |
Assets, fair value | 50,208,000 | 2,961,000
Natural gas forward contract | Cash flow hedges | other current liabilities |  |
Disclosure of financial assets [line items] |  |
Liabilities, fair value | (3,961,000) | (14,758,000)
Natural gas forward contract | Cash flow hedges | Other non-current liabilities |  |
Disclosure of financial assets [line items] |  |
Liabilities, fair value | $ (56,138,000) | $ (166,039,000)
Natural gas forward contract | Cash flow hedges | Bottom of range |  |
Disclosure of financial assets [line items] |  |
Hedging instrument, daily energy (in mmbtu) | MMBTU | 50 | 50
Natural gas forward contract | Cash flow hedges | Top of range |  |
Disclosure of financial assets [line items] |  |
Hedging instrument, daily energy (in mmbtu) | MMBTU | 130 | 90

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