Fair Value Measurements (Summary of Fair Value of Derivative Assets and Liabilities and Effect of Counterparty Netting) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets
Total Fair Value of Gross Assets	$ 1,401	$ 1,342
Effect of Counterparty Netting	(1,241)	(1,188)
Effect of Collateral Netting	0	0
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet	160	154
Liabilities
Total Fair Value Gross Liabilities	17,242	14,195
Effect of Counterparty Netting	(1,241)	(1,188)
Effect of Collateral Netting	(168)	(80)
Difference in Carrying Value and Fair Value	(1,643)	(1,438)
Net Carrying Value Presented on the Balance Sheet	14,190	11,489
Level 1
Assets
Total Fair Value of Gross Assets	979	947
Liabilities
Total Fair Value Gross Liabilities	986	884
Level 2
Assets
Total Fair Value of Gross Assets	422	395
Liabilities
Total Fair Value Gross Liabilities	16,255	13,311
Level 3
Assets
Total Fair Value of Gross Assets	0	0
Liabilities
Total Fair Value Gross Liabilities	1	0
Commodity Derivatives \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets	1,242	1,188
Effect of Counterparty Netting	(1,241)	(1,188)
Effect of Collateral Netting	0	0
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet	1	0
Liabilities
Commodity Derivative Liabilities	1,410	1,269
Effect of Counterparty Netting	(1,241)	(1,188)
Effect of Collateral Netting	(168)	(80)
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet	1	1
Commodity Derivatives \| OTC instruments
Liabilities
Commodity Derivative Liabilities		1
Effect of Counterparty Netting		0
Effect of Collateral Netting		0
Difference in Carrying Value and Fair Value		0
Net Carrying Value Presented on the Balance Sheet		1
Commodity Derivatives \| Physical forward contracts
Assets
Total Fair Value of Gross Assets	30	26
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	30	26
Liabilities
Commodity Derivative Liabilities	15	12
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	15	12
Commodity Derivatives \| Level 1 \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets	850	820
Liabilities
Commodity Derivative Liabilities	986	884
Commodity Derivatives \| Level 1 \| OTC instruments
Liabilities
Commodity Derivative Liabilities		0
Commodity Derivatives \| Level 1 \| Physical forward contracts
Assets
Total Fair Value of Gross Assets	0	0
Liabilities
Commodity Derivative Liabilities	0	0
Commodity Derivatives \| Level 2 \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets	392	368
Liabilities
Commodity Derivative Liabilities	424	385
Commodity Derivatives \| Level 2 \| OTC instruments
Liabilities
Commodity Derivative Liabilities		1
Commodity Derivatives \| Level 2 \| Physical forward contracts
Assets
Total Fair Value of Gross Assets	30	26
Liabilities
Commodity Derivative Liabilities	14	12
Commodity Derivatives \| Level 3 \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets	0	0
Liabilities
Commodity Derivative Liabilities	0	0
Commodity Derivatives \| Level 3 \| OTC instruments
Liabilities
Commodity Derivative Liabilities		0
Commodity Derivatives \| Level 3 \| Physical forward contracts
Assets
Total Fair Value of Gross Assets	0	0
Liabilities
Commodity Derivative Liabilities	1	0
Interest rate derivatives
Assets
Interest rate derivatives		1
Effect of Counterparty Netting		0
Effect of Collateral Netting		0
Difference in Carrying Value and Fair Value		0
Net Carrying Value Presented on the Balance Sheet		1
Liabilities
Interest rate derivatives	7
Effect of Counterparty Netting	0
Effect of Collateral Netting	0
Difference in Carrying Value and Fair Value	0
Net Carrying Value Presented on the Balance Sheet	7
Interest rate derivatives \| Level 1
Assets
Interest rate derivatives		0
Liabilities
Interest rate derivatives	0
Interest rate derivatives \| Level 2
Assets
Interest rate derivatives		1
Liabilities
Interest rate derivatives	7
Interest rate derivatives \| Level 3
Assets
Interest rate derivatives		0
Liabilities
Interest rate derivatives	0
Rabbi trust assets
Assets
Rabbi trust assets	129	127
Difference in Carrying Value and Fair Value	0	0
Rabbi trust assets \| Level 1
Assets
Rabbi trust assets	129	127
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	1,787	1,100
Difference in Carrying Value and Fair Value	0	0
Floating-rate debt \| Net Carrying Value Presented on the Balance Sheet
Liabilities
Debt	1,787	1,100
Floating-rate debt \| Level 1
Liabilities
Debt	0	0
Floating-rate debt \| Level 2
Liabilities
Debt	1,787	1,100
Floating-rate debt \| Level 3
Liabilities
Debt	0	0
Fixed-rate debt, excluding finance leases
Liabilities
Debt	14,023	11,813
Difference in Carrying Value and Fair Value	(1,643)	(1,438)
Fixed-rate debt, excluding finance leases \| Net Carrying Value Presented on the Balance Sheet
Liabilities
Debt	12,380	10,375
Fixed-rate debt, excluding finance leases \| Level 1
Liabilities
Debt	0	0
Fixed-rate debt, excluding finance leases \| Level 2
Liabilities
Debt	14,023	11,813
Fixed-rate debt, excluding finance leases \| Level 3
Liabilities
Debt	$ 0	$ 0

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