Fair Value Measurements (Summary of Fair Value of Derivative Assets and Liabilities and Effect of Counterparty Netting) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Assets
Effect of Counterparty Netting	$ (945)	$ (1,075)
Effect of Collateral Netting		(89)
Liabilities
Effect of Counterparty Netting	(945)	(1,075)
Effect of Collateral Netting	(22)	0
Difference in Carrying Value and Fair Value	(998)	49
Fixed-rate debt, excluding capital leases
Liabilities
Difference in Carrying Value and Fair Value	(998)	49
Total Fair Value of Gross Assets & Liabilities
Assets
Assets, fair value disclosure	1,116	1,383
Liabilities
Total liabilities, fair value disclosure gross	13,253	12,027
Total Fair Value of Gross Assets & Liabilities \| Floating-rate debt
Liabilities
Debt, fair value gross	1,475	1,200
Total Fair Value of Gross Assets & Liabilities \| Fixed-rate debt, excluding capital leases
Liabilities
Debt, fair value gross	10,778	9,727
Total Fair Value of Gross Assets & Liabilities \| Rabbi trust assets
Assets
Rabbi trust assets	122	104
Net Carrying Value Presented on the Balance Sheet
Assets
Assets, fair value disclosure	171	219
Liabilities
Total liabilities, fair value disclosure gross	11,288	11,001
Net Carrying Value Presented on the Balance Sheet \| Floating-rate debt
Liabilities
Debt, fair value gross	1,475	1,200
Net Carrying Value Presented on the Balance Sheet \| Fixed-rate debt, excluding capital leases
Liabilities
Debt, fair value gross	9,780	9,776
Net Carrying Value Presented on the Balance Sheet \| Rabbi trust assets
Assets
Rabbi trust assets	122	104
Interest rate derivatives \| Total Fair Value of Gross Assets & Liabilities
Assets
Interest rate derivatives	3	15
Interest rate derivatives \| Net Carrying Value Presented on the Balance Sheet
Assets
Interest rate derivatives	3	15
Exchange-cleared instruments
Assets
Effect of Counterparty Netting	(945)	(1,075)
Effect of Collateral Netting	0	(89)
Liabilities
Effect of Counterparty Netting	(945)	(1,075)
Effect of Collateral Netting	(22)	0
Exchange-cleared instruments \| Total Fair Value of Gross Assets & Liabilities
Assets
Commodity derivative assets, fair value gross	954	1,221
Liabilities
Commodity derivative liabilities, fair value gross	968	1,077
Exchange-cleared instruments \| Net Carrying Value Presented on the Balance Sheet
Assets
Commodity derivative assets, fair value gross	9	57
Liabilities
Commodity derivative liabilities, fair value gross	1	2
Physical forward contracts
Liabilities
Effect of Counterparty Netting	0
Physical forward contracts \| Total Fair Value of Gross Assets & Liabilities
Assets
Commodity derivative assets, fair value gross	37	43
Liabilities
Commodity derivative liabilities, fair value gross	32	20
Physical forward contracts \| Net Carrying Value Presented on the Balance Sheet
Assets
Commodity derivative assets, fair value gross	37	43
Liabilities
Commodity derivative liabilities, fair value gross	32	20
OTC instruments \| Total Fair Value of Gross Assets & Liabilities
Liabilities
Commodity derivative liabilities, fair value gross		3
OTC instruments \| Net Carrying Value Presented on the Balance Sheet
Liabilities
Commodity derivative liabilities, fair value gross		3
Level 1
Assets
Assets, fair value disclosure	623	778
Liabilities
Total liabilities, fair value disclosure gross	519	605
Level 1 \| Floating-rate debt
Liabilities
Debt, fair value gross	0	0
Level 1 \| Fixed-rate debt, excluding capital leases
Liabilities
Debt, fair value gross	0	0
Level 1 \| Rabbi trust assets
Assets
Rabbi trust assets	122	104
Level 1 \| Interest rate derivatives
Assets
Interest rate derivatives	0	0
Level 1 \| Exchange-cleared instruments
Assets
Commodity derivative assets, fair value gross	501	674
Liabilities
Commodity derivative liabilities, fair value gross	519	605
Level 1 \| Physical forward contracts
Assets
Commodity derivative assets, fair value gross	0	0
Liabilities
Commodity derivative liabilities, fair value gross	0	0
Level 1 \| OTC instruments
Liabilities
Commodity derivative liabilities, fair value gross		0
Level 2
Assets
Assets, fair value disclosure	490	601
Liabilities
Total liabilities, fair value disclosure gross	12,734	11,422
Level 2 \| Floating-rate debt
Liabilities
Debt, fair value gross	1,475	1,200
Level 2 \| Fixed-rate debt, excluding capital leases
Liabilities
Debt, fair value gross	10,778	9,727
Level 2 \| Rabbi trust assets
Assets
Rabbi trust assets	0	0
Level 2 \| Interest rate derivatives
Assets
Interest rate derivatives	3	15
Level 2 \| Exchange-cleared instruments
Assets
Commodity derivative assets, fair value gross	453	547
Liabilities
Commodity derivative liabilities, fair value gross	449	472
Level 2 \| Physical forward contracts
Assets
Commodity derivative assets, fair value gross	34	39
Liabilities
Commodity derivative liabilities, fair value gross	32	20
Level 2 \| OTC instruments
Liabilities
Commodity derivative liabilities, fair value gross		3
Level 3
Assets
Assets, fair value disclosure	3	4
Liabilities
Total liabilities, fair value disclosure gross	0	0
Level 3 \| Floating-rate debt
Liabilities
Debt, fair value gross	0	0
Level 3 \| Fixed-rate debt, excluding capital leases
Liabilities
Debt, fair value gross	0	0
Level 3 \| Rabbi trust assets
Assets
Rabbi trust assets	0	0
Level 3 \| Interest rate derivatives
Assets
Interest rate derivatives	0	0
Level 3 \| Exchange-cleared instruments
Assets
Commodity derivative assets, fair value gross	0	0
Liabilities
Commodity derivative liabilities, fair value gross	0	0
Level 3 \| Physical forward contracts
Assets
Commodity derivative assets, fair value gross	3	4
Liabilities
Commodity derivative liabilities, fair value gross	$ 0	0
Level 3 \| OTC instruments
Liabilities
Commodity derivative liabilities, fair value gross		$ 0

The amount by which the aggregate fair value of the liability exceeds (or is less than) the carrying value.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value, after effects of master netting arrangements, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset offset against an obligation to return collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after effects of master netting arrangements, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset offset against the right to receive collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value as of the balance sheet date of all interest rate derivative assets designated as cash flow hedging instruments.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair value of financial and nonfinancial obligations.

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Fair value as of the balance sheet date of all price risk derivative assets not designated as hedging instruments.

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Fair value as of the balance sheet date of all price risk derivative liabilities not designated as hedging instruments.

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