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Financial instruments - Summary of Commodity Volumes Associated with Derivative Contracts (Detail)
Dec. 31, 2021
MMBTU
Derivative [Line Items]  
Commodity volumes, Gas 3,405,544
Financial contracts: Swaps  
Derivative [Line Items]  
Commodity volumes, Gas 3,239,873
Options  
Derivative [Line Items]  
Commodity volumes, Gas 165,671