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Financial instruments - Summary of Commodity Volumes Associated with Derivative Contracts (Detail)
Dec. 31, 2020
MMBTU
Derivative [Line Items]  
Commodity volumes, Gas 3,810,544
Swap  
Derivative [Line Items]  
Commodity volumes, Gas 1,830,852
Options  
Derivative [Line Items]  
Commodity volumes, Gas 479,692
Forward contracts  
Derivative [Line Items]  
Commodity volumes, Gas 1,500,000