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Accounting for Derivative Instruments and Hedging Activities - Net Notional Volume Buy/Sell of Open Derivative Transactions (Details)
shares in Millions, bbl in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
MWh
MMBTU
T
shares
bbl
Dec. 31, 2016
USD ($)
MWh
MMBTU
T
shares
bbl
Long | Emissions    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, mass (ton) | T 1 0
Long | Coal    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, mass (ton) | T 21 35
Long | Oil    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, volume (in barrels) | bbl 0 1
Long | Power    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh 14 7
Long | Interest    
Derivative [Line Items]    
Derivative, notional amount (in usd) | $ $ 3,876 $ 3,429
Long | Equity    
Derivative [Line Items]    
Derivative, non-monetary notional amount (in shares) | shares 1 1
Short | Natural Gas    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MMBTU 17 53
Short | Capacity    
Derivative [Line Items]    
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh 1 1