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Risk Management - Credit Risks (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Credit Derivatives [Line Items]    
Additional Collateral, Aggregate Fair Value $ 6  
Energy commodity derivative contracts    
Credit Derivatives [Line Items]    
Letters of credit outstanding 0 $ 0
Initial Margin Requirements 11  
Variation Margin Requirements 8  
Other Current Liabilities | Contract and Over the Counter | Energy commodity derivative contracts    
Credit Derivatives [Line Items]    
Derivative, Collateral, Obligation to Return Cash $ 3 $ 15