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Derivative Accounting - Schedule of Gross Notional Amounts Outstanding (Details) - Commodity Contracts
GWh in Thousands, Bcf in Thousands
Sep. 30, 2021
GWh
Bcf
Dec. 31, 2020
GWh
Bcf
Outstanding gross notional amount of derivatives    
Power | GWh 0 368
Gas | Bcf 161 205