Risk Management - Credit Risks (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Additional Collateral, Aggregate Fair Value	$ 11
Energy commodity derivative contracts
Credit Derivatives [Line Items]
Letters of Credit Outstanding, Amount	0
Initial Margin Requirements	6
Variation Margin Requirements	21
Other Current Liabilities | Contract and Over the Counter | Energy commodity derivative contracts
Credit Derivatives [Line Items]
Derivative, Collateral, Obligation to Return Cash	$ 15	$ 16