Fair Value Measurements (Schedule Of Carrying Value And Fair Value Measurement Information For Financial Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, assets	$ 212	$ 11
Derivatives, liabilities	(358)	(244)
Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,533	1,542
Debt	(10,406)	(10,154)
Installment payment	(250)	(473)
Capital lease obligations	(4)	(7)
Carrying Amount [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, assets	211	10
Derivatives, liabilities	(294)	(203)
Carrying Amount [Member] | Interest Rate Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, assets	1	1
Derivatives, liabilities	(64)	(41)
Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,533	1,542
Debt	(11,782)	(10,760)
Installment payment	(250)	(477)
Capital lease obligations	(3)	(6)
Total Fair Value [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, assets	211	10
Derivatives, liabilities	(294)	(203)
Total Fair Value [Member] | Interest Rate Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, assets	1	1
Derivatives, liabilities	(64)	(41)
Level 1 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,454	1,298
Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	79	244
Debt	(11,782)	(10,760)
Installment payment	(250)	(477)
Capital lease obligations	(3)	(6)
Level 2 Inputs [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, assets	211	10
Derivatives, liabilities	(294)	(203)
Level 2 Inputs [Member] | Interest Rate Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, assets	1	1
Derivatives, liabilities	$ (64)	$ (41)