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Financial instruments - Natural gas forward contracts (Details) - Natural gas forward contract | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) $ / MMBTU | Dec. 31, 2017 USD ($) $ / MMBTU
Disclosure of financial assets [line items] |  |
Hedge ineffectiveness | $ 0 |
Cash flow hedges |  |
Disclosure of financial assets [line items] |  |
Notional amount | 426,000,000 | $ 473,000,000
Negative fair value | $ 105,700,000 | $ 90,200,000
Cash flow hedges | Geismar 2 Facility |  |
Disclosure of financial assets [line items] |  |
Percentage of gas requirement for forward contracts that will be managed for changes in prices | 40.00% |
Average contract price (in usd per mmbtu) | $ / MMBTU | 3.81 | 3.74
Remaining term | 7 years |
Cash flow hedges | Medicine Hat Facility |  |
Disclosure of financial assets [line items] |  |
Average contract price (in usd per mmbtu) | $ / MMBTU | 1.96 | 1.96

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