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Financial instruments - Long-term Energy Derivative Contracts (Detail) - Cash flow hedge
Dec. 31, 2017
MWh
$ / MWh
PJM Western HUB  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 688,147
Receive average prices (per MW-hr) | $ / MWh 40.40
NI HUB  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 2,926,922
Receive average prices (per MW-hr) | $ / MWh 29.26
ERCOT North HUB  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 3,330,876
Receive average prices (per MW-hr) | $ / MWh 36.46