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Financial instruments - Natural gas forward contracts (Details)
 12 Months Ended
Dec. 31, 2019
USD ($)
$ / MMBTU
Dec. 31, 2018
USD ($)
$ / MMBTU
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%  
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]    
Notional amount | $ 970,000,000 $ 426,000,000
Negative fair value | $ $ 195,100,000 $ 105,700,000
Natural gas forward contract | Cash flow hedges | Geismar 2 Facility    
Disclosure of financial assets [line items]    
Average contract price (in usd per mmbtu) | $ / MMBTU 3.89 3.81
Remaining term 6 years  
Natural gas forward contract | Cash flow hedges | Geismar 3 Facility    
Disclosure of financial assets [line items]    
Average contract price (in usd per mmbtu) | $ / MMBTU 3.24  
Remaining term 10 years  
Natural gas forward contract | Cash flow hedges | Medicine Hat Facility    
Disclosure of financial assets [line items]    
Average contract price (in usd per mmbtu) | $ / MMBTU 1.96 1.96