Fair Value Measurements (Schedule Of Fair Value, Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Net, Current	$ 205
Risk Management Assets, Net, Long-term	246	$ 16
Risk Management Liabilities, Net, Current	236	254
Risk Management Liabilities, Net, Long-term	13	35
Guarantor Obligations, Current Carrying Value	19	19
Accounts Payable and Accrued Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guarantor Obligations, Current Carrying Value	5	5
Accounts Payable and Accrued Liabilities [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guarantor Obligations, Current Carrying Value	5	5
Accounts Payable and Accrued Liabilities [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guarantor Obligations, Current Carrying Value	5	5
Other Liabilities and Provisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guarantor Obligations, Current Carrying Value	14	14
Other Liabilities and Provisions [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guarantor Obligations, Current Carrying Value	14	14
Other Liabilities and Provisions [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guarantor Obligations, Current Carrying Value	14	14
Level 2 [Member] | Accounts Payable and Accrued Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guarantor Obligations, Current Carrying Value	5	5
Level 2 [Member] | Other Liabilities and Provisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guarantor Obligations, Current Carrying Value	14	14
Commodity Contract [Member] | Other Current Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Gross Liabilities	(15)	(11)
Commodity Contract [Member] | Other Current Assets [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Gross assets	189	11
Commodity Contract [Member] | Other Current Assets [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Net, Current	174
Commodity Contract [Member] | Other Noncurrent Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Gross Liabilities	(2)	(3)
Commodity Contract [Member] | Other Noncurrent Assets [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Gross assets	248	19
Commodity Contract [Member] | Other Noncurrent Assets [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Net, Long-term	246	16
Commodity Contract [Member] | Other Current Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross Assets	(15)	(11)
Commodity Contract [Member] | Other Current Liabilities [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross liabilities	250	264
Commodity Contract [Member] | Other Current Liabilities [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Net, Current	235	253
Commodity Contract [Member] | Other Liabilities and Provisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross Assets	(2)	(3)
Commodity Contract [Member] | Other Liabilities and Provisions [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross liabilities	15	38
Commodity Contract [Member] | Other Liabilities and Provisions [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Net, Long-term	13	35
Commodity Contract [Member] | Level 1 [Member] | Other Current Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross liabilities	3
Commodity Contract [Member] | Level 2 [Member] | Other Current Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Gross assets	189	11
Commodity Contract [Member] | Level 2 [Member] | Other Noncurrent Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Gross assets	248	19
Commodity Contract [Member] | Level 2 [Member] | Other Current Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross liabilities	196	228
Commodity Contract [Member] | Level 2 [Member] | Other Liabilities and Provisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross liabilities	15	38
Commodity Contract [Member] | Level 3 [Member] | Other Current Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross liabilities	51	36
Foreign Exchange Contract [Member] | Other Current Assets [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Gross assets	31
Foreign Exchange Contract [Member] | Other Current Assets [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Net, Current	31
Foreign Exchange Contract [Member] | Other Current Liabilities [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross liabilities	1	1
Foreign Exchange Contract [Member] | Other Current Liabilities [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Net, Current	1	1
Foreign Exchange Contract [Member] | Level 2 [Member] | Other Current Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Assets, Gross assets	31
Foreign Exchange Contract [Member] | Level 2 [Member] | Other Current Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Management Liabilities, Gross liabilities	$ 1	$ 1