Derivatives and Financial Instruments (Summary of Commodity Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets
Liabilities	$ (1,188)	$ (1,075)
Effect of Collateral Netting	0	(89)
Liabilities
Assets	1,188	1,075
Effect of Collateral Netting	80	0
Not Designated as Hedging Instrument | Commodity Derivatives
Liabilities
Effect of Collateral Netting	80	(89)
Total
Assets	1,214	1,264
Liabilities	(1,282)	(1,100)
Net Carrying Value Presented on the Balance Sheet	12	75
Not Designated as Hedging Instrument | Commodity Derivatives | Prepaid expenses and other current assets
Assets
Assets	23	1,257
Liabilities	0	(1,070)
Effect of Collateral Netting	0	(89)
Net Carrying Value Presented on the Balance Sheet	23	98
Not Designated as Hedging Instrument | Commodity Derivatives | Other assets
Assets
Assets	3	2
Liabilities	0	0
Effect of Collateral Netting	0	0
Net Carrying Value Presented on the Balance Sheet	3	2
Not Designated as Hedging Instrument | Commodity Derivatives | Other accruals
Liabilities
Assets	1,188	0
Liabilities	(1,281)	(23)
Effect of Collateral Netting	80	0
Net Carrying Value Presented on the Balance Sheet	13	23
Not Designated as Hedging Instrument | Commodity Derivatives | Other liabilities and deferred credits
Liabilities
Assets	0	5
Liabilities	(1)	(7)
Effect of Collateral Netting	0	0
Net Carrying Value Presented on the Balance Sheet	$ 1	$ 2