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Financial instruments - Long-term Energy Derivative Contracts (Detail) - Cash flow hedge
Dec. 31, 2019
MWh
$ / MWh
PJM Western HUB  
Derivative [Line Items]  
Receive average prices (per MW-hr) | $ / MWh 35.35
Notional quantity (MW-hrs) | MWh 757,075
PJM NI HUB  
Derivative [Line Items]  
Receive average prices (per MW-hr) | $ / MWh 25.54
Notional quantity (MW-hrs) | MWh 3,443,530
ERCOT North HUB  
Derivative [Line Items]  
Receive average prices (per MW-hr) | $ / MWh 36.46
Notional quantity (MW-hrs) | MWh 2,665,068