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Financial instruments - Summary of Commodity Volumes Associated with Derivative Contracts (Detail)
Dec. 31, 2017
MMBTU
Derivative [Line Items]  
Commodity volumes, Gas 15,457,678
Swap  
Derivative [Line Items]  
Commodity volumes, Gas 2,518,812
Options  
Derivative [Line Items]  
Commodity volumes, Gas 518,866
Forward contracts  
Derivative [Line Items]  
Commodity volumes, Gas 12,420,000