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Accounting for Derivative Instruments and Hedging Activities - Net notional volume buy/sell of open derivative transactions (Details)
certificates in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Billions
9 Months Ended 12 Months Ended
Sep. 30, 2020
MMBTU
MWh
certificates
T
Dec. 31, 2019
MMBTU
MWh
certificates
T
Nov. 05, 2020
USD ($)
Interest Rate Swap | Subsequent Event      
Derivative [Line Items]      
Derivative, notional amount | $     $ 1.6
Long | Emissions      
Volumetric Underlying Derivative Transactions      
Derivative, nonmonetary notional amount, mass (ton) | T 1 3  
Long | Renewable Energy Certificates      
Volumetric Underlying Derivative Transactions      
Derivative, non-monetary notional amount (in shares) | certificates 3 1  
Long | Coal      
Volumetric Underlying Derivative Transactions      
Derivative, nonmonetary notional amount, mass (ton) | T 4 10  
Long | Power      
Volumetric Underlying Derivative Transactions      
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MWh (51) (38)  
Short | Natural Gas      
Volumetric Underlying Derivative Transactions      
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MMBTU (264) (181)  
Short | Capacity      
Volumetric Underlying Derivative Transactions      
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MMBTU (1) (1)