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Financial instruments - Long-term Energy Derivative Contracts (Detail) - Cash flow hedge
Jun. 30, 2020
MWh
$ / MWh
$ / MWh
PJM Western HUB  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 692,015
Receive average prices (per MW-hr) | $ / MWh 34.65
NI HUB  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 3,170,648
Receive average prices (per MW-hr) | $ / MWh 25.13
ERCORT North HUB  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 2,469,582
Receive average prices (per MW-hr) | $ / MWh 36.46