Fair Value Measurements (Summary of Fair Value of Derivative Assets and Liabilities and Effect of Counterparty Netting) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets
Total Fair Value of Gross Assets	$ 6,952	$ 1,342
Effect of Counterparty Netting	(6,179)	(1,188)
Effect of Collateral Netting	(455)	0
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet	318	154
Liabilities
Total Fair Value Gross Liabilities	18,880	14,195
Effect of Counterparty Netting	(6,179)	(1,188)
Effect of Collateral Netting	0	(80)
Difference in Carrying Value and Fair Value	17	(1,438)
Net Carrying Value Presented on the Balance Sheet	12,718	11,489
Interest rate derivatives
Liabilities
Net Carrying Value Presented on the Balance Sheet	8
Level 1
Assets
Total Fair Value of Gross Assets	5,367	947
Liabilities
Total Fair Value Gross Liabilities	4,589	884
Level 2
Assets
Total Fair Value of Gross Assets	1,585	395
Liabilities
Total Fair Value Gross Liabilities	14,291	13,311
Level 3
Liabilities
Total Fair Value Gross Liabilities	0	0
Commodity Derivatives
Assets
Effect of Collateral Netting	0	0
Liabilities
Effect of Collateral Netting	0	0
Difference in Carrying Value and Fair Value	0	0
Commodity Derivatives \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets	6,796	1,188
Effect of Counterparty Netting	(6,179)	(1,188)
Effect of Collateral Netting	(455)	0
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet	162
Liabilities
Commodity Derivative Liabilities	6,179	1,269
Effect of Counterparty Netting	(6,179)	(1,188)
Effect of Collateral Netting	0	(80)
Net Carrying Value Presented on the Balance Sheet	0	1
Commodity Derivatives \| Physical forward contracts
Assets
Total Fair Value of Gross Assets	40	26
Effect of Counterparty Netting	0	0
Difference in Carrying Value and Fair Value	0	0
Net Carrying Value Presented on the Balance Sheet	40	26
Liabilities
Commodity Derivative Liabilities	32	12
Effect of Counterparty Netting	0	0
Net Carrying Value Presented on the Balance Sheet	32	12
Commodity Derivatives \| Physical forward contracts
Liabilities
Commodity Derivative Liabilities	4	1
Effect of Counterparty Netting	0	0
Net Carrying Value Presented on the Balance Sheet	4	1
Commodity Derivatives \| Level 1 \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets	5,251	820
Liabilities
Commodity Derivative Liabilities	4,589	884
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 1 \| Physical forward contracts
Liabilities
Commodity Derivative Liabilities	0	0
Commodity Derivatives \| Level 2 \| Exchange-cleared instruments
Assets
Total Fair Value of Gross Assets	1,545	368
Liabilities
Commodity Derivative Liabilities	1,590	385
Commodity Derivatives \| Level 2 \| Physical forward contracts
Assets
Total Fair Value of Gross Assets	40	26
Liabilities
Commodity Derivative Liabilities	32	12
Commodity Derivatives \| Level 2 \| Physical forward contracts
Liabilities
Commodity Derivative Liabilities	4	1
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 \| Physical forward contracts
Liabilities
Commodity Derivative Liabilities	0	0
Commodity Derivatives \| Level 3 \| Physical forward contracts
Liabilities
Commodity Derivative Liabilities	0	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
Interest rate derivatives \| Level 1
Assets
Interest rate derivatives		0
Interest rate derivatives \| Level 2
Assets
Interest rate derivatives		1
Interest rate derivatives \| Level 3
Assets
Interest rate derivatives		0
Rabbi trust assets
Assets
Rabbi trust assets	116	127
Rabbi trust assets \| Level 1
Assets
Rabbi trust assets	116	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	2,097	1,100
Floating-rate debt \| Net Carrying Value Presented on the Balance Sheet
Liabilities
Debt	2,097	1,100
Floating-rate debt \| Level 1
Liabilities
Debt	0	0
Floating-rate debt \| Level 2
Liabilities
Debt	2,097	1,100
Floating-rate debt \| Level 3
Liabilities
Debt	0	0
Fixed-rate debt, excluding capital leases
Liabilities
Debt	10,560	11,813
Difference in Carrying Value and Fair Value	17	(1,438)
Fixed-rate debt, excluding capital leases \| Net Carrying Value Presented on the Balance Sheet
Liabilities
Debt	10,577	10,375
Fixed-rate debt, excluding capital leases \| Level 1
Liabilities
Debt	0	0
Fixed-rate debt, excluding capital leases \| Level 2
Liabilities
Debt	10,560	11,813
Fixed-rate debt, excluding capital leases \| Level 3
Liabilities
Debt	0	$ 0
Derivative Financial Instruments, Liabilities \| Interest rate derivatives
Liabilities
Interest rate derivatives	8
Derivative Financial Instruments, Liabilities \| Fair Value, Recurring \| Level 1 \| Interest rate derivatives
Liabilities
Interest rate derivatives	0
Derivative Financial Instruments, Liabilities \| Fair Value, Recurring \| Level 2 \| Interest rate derivatives
Liabilities
Interest rate derivatives	8
Derivative Financial Instruments, Liabilities \| Fair Value, Recurring \| Level 3 \| Interest rate derivatives
Liabilities
Interest rate derivatives	$ 0

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