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Accounting for Derivative Instruments and Hedging Activities - Net Notional Volume Buy/Sell of Open Derivative Transactions (Details)
shares in Millions, T in Millions, MWh in Millions, MMBTU in Millions
 12 Months Ended
Dec. 31, 2020
MWh
MMBTU
T
shares
Dec. 31, 2019
MWh
MMBTU
T
shares
Long | Emissions    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, mass (ton) | T 1 3
Long | Renewables Energy Certificates    
Derivative [Line Items]    
Derivative, non-monetary notional amount (in shares) | shares 5 1
Long | Coal    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, mass (ton) | T 2 10
Long | Power    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh 57 38
Short | Natural Gas    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MMBTU 286 181
Short | Capacity    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh 1 1