Fair Value Measurements (Summary of Fair Value of Derivative Assets and Liabilities and Effect of Counterparty Netting) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets
Total Fair Value of Gross Assets & Liabilities	$ 1,342	$ 1,383
Effect of Counterparty Netting	(1,188)	(1,075)
Effect of Collateral Netting	0	(89)
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet	154	219
Liabilities
Financial and Nonfinancial Liabilities, Fair Value Disclosure	14,195	12,027
Effect of Counterparty Netting	(1,188)	(1,075)
Effect of Collateral Netting	(80)	0
Difference in Carrying Value and Fair Value	(1,438)	49
Net Carrying Value Presented on the Balance Sheet	11,489	11,001
Level 1
Assets
Total Fair Value of Gross Assets & Liabilities	947	778
Liabilities
Financial and Nonfinancial Liabilities, Fair Value Disclosure	884	605
Level 2
Assets
Total Fair Value of Gross Assets & Liabilities	395	601
Liabilities
Financial and Nonfinancial Liabilities, Fair Value Disclosure	13,311	11,422
Level 3
Assets
Total Fair Value of Gross Assets & Liabilities		4
Liabilities
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	0
Commodity Derivative Assets
Assets
Effect of Collateral Netting	0	0
Liabilities
Effect of Collateral Netting	0	0
Difference in Carrying Value and Fair Value	0	0
Commodity Derivative Assets \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets & Liabilities	1,188	1,221
Effect of Counterparty Netting	(1,188)	(1,075)
Effect of Collateral Netting	0	(89)
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet		57
Liabilities
Total Fair Value of Gross Assets & Liabilities	1,269	1,077
Effect of Counterparty Netting	(1,188)	(1,075)
Effect of Collateral Netting	(80)	0
Net Carrying Value Presented on the Balance Sheet	1	2
Commodity Derivative Assets \| OTC instruments
Liabilities
Total Fair Value of Gross Assets & Liabilities	1	3
Effect of Counterparty Netting	0	0
Net Carrying Value Presented on the Balance Sheet	1	3
Commodity Derivative Assets \| Physical forward contracts
Assets
Total Fair Value of Gross Assets & Liabilities	26	43
Effect of Counterparty Netting	0	0
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet	26	43
Liabilities
Total Fair Value of Gross Assets & Liabilities	12	20
Effect of Counterparty Netting	0	0
Net Carrying Value Presented on the Balance Sheet	12	20
Commodity Derivative Assets \| Level 1 \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets & Liabilities	820	674
Liabilities
Total Fair Value of Gross Assets & Liabilities	884	605
Commodity Derivative Assets \| Level 1 \| OTC instruments
Liabilities
Total Fair Value of Gross Assets & Liabilities	0	0
Commodity Derivative Assets \| Level 1 \| Physical forward contracts
Assets
Total Fair Value of Gross Assets & Liabilities	0	0
Liabilities
Total Fair Value of Gross Assets & Liabilities	0	0
Commodity Derivative Assets \| Level 2 \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets & Liabilities	368	547
Liabilities
Total Fair Value of Gross Assets & Liabilities	385	472
Commodity Derivative Assets \| Level 2 \| OTC instruments
Liabilities
Total Fair Value of Gross Assets & Liabilities	1	3
Commodity Derivative Assets \| Level 2 \| Physical forward contracts
Assets
Total Fair Value of Gross Assets & Liabilities	26	39
Liabilities
Total Fair Value of Gross Assets & Liabilities	12	20
Commodity Derivative Assets \| Level 3 \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets & Liabilities	0	0
Liabilities
Total Fair Value of Gross Assets & Liabilities	0	0
Commodity Derivative Assets \| Level 3 \| OTC instruments
Liabilities
Total Fair Value of Gross Assets & Liabilities	0	0
Commodity Derivative Assets \| Level 3 \| Physical forward contracts
Assets
Total Fair Value of Gross Assets & Liabilities		4
Liabilities
Total Fair Value of Gross Assets & Liabilities	0	0
Interest rate derivatives
Assets
Total Fair Value of Gross Assets & Liabilities	1	15
Effect of Counterparty Netting	0	0
Effect of Collateral Netting	0	0
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet	1	15
Interest rate derivatives \| Level 1
Assets
Total Fair Value of Gross Assets & Liabilities	0	0
Interest rate derivatives \| Level 2
Assets
Total Fair Value of Gross Assets & Liabilities	1	15
Interest rate derivatives \| Level 3
Assets
Total Fair Value of Gross Assets & Liabilities	0	0
Rabbi trust assets
Assets
Rabbi trust assets	127	104
Rabbi trust assets \| Level 1
Assets
Rabbi trust assets	127	104
Rabbi trust assets \| Level 2
Assets
Rabbi trust assets	0	0
Rabbi trust assets \| Level 3
Assets
Rabbi trust assets	0	0
Floating-rate debt
Liabilities
Debt	1,100	1,200
Floating-rate debt \| Level 1
Liabilities
Debt	0	0
Floating-rate debt \| Level 2
Liabilities
Debt	1,100	1,200
Floating-rate debt \| Level 3
Liabilities
Debt	0	0
Fixed-rate debt, excluding finance leases
Liabilities
Debt	11,813	9,727
Difference in Carrying Value and Fair Value	(1,438)	49
Fixed-rate debt, excluding finance leases \| Level 1
Liabilities
Debt	0	0
Fixed-rate debt, excluding finance leases \| Level 2
Liabilities
Debt	11,813	9,727
Fixed-rate debt, excluding finance leases \| Level 3
Liabilities
Debt	0	0
Reported Value Measurement \| Floating-rate debt
Liabilities
Debt	1,100	1,200
Reported Value Measurement \| Fixed-rate debt, excluding finance leases
Liabilities
Debt	$ 10,375	$ 9,776

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