Financial Instruments And Risk Management (Unrealized Risk Management Position) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Risk Management, Current asset	$ 205
Risk Management, Long-term asset	246	$ 16
Risk Management, Total asset	451	16
Risk Management, Current liability	236	254
Risk Management, Long-term liability	13	35
Risk Management, Total liability	249	289
Guarantor Obligations, Current Carrying Value	19	19
Risk Management and Other Derivative Guarantee, at Fair Value, Net	183	(292)
Accounts Payable and Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Guarantor Obligations, Current Carrying Value	5	5
Other Liabilities and Provisions [Member]
Derivatives, Fair Value [Line Items]
Guarantor Obligations, Current Carrying Value	$ 14	$ 14