XML 137 R121.htm IDEA: XBRL DOCUMENT v3.10.0.1
Financial instruments - Long-term Energy Derivative Contracts (Detail) - Cash flow hedge
Dec. 31, 2018
MWh
$ / MWh
PJM Western HUB  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 871,391
Receive average prices (per MW-hr) | $ / MWh 36.33
PJM NI HUB  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 2,438,697
Receive average prices (per MW-hr) | $ / MWh 29.06
ERCOT North HUB  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 2,997,939
Receive average prices (per MW-hr) | $ / MWh 36.46