Financial Instruments and Risk Management (Recognized Gains and Losses for Derivative Instruments Not Designated as Hedging Instruments) (Details) - Nondesignated [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss)	$ 9,567	$ (36,698)
Commodity Derivative Contracts [Member] | Sales And Other Operating Revenues [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss)	$ 9,567	(63,412)
Foreign Exchange Derivative Contracts [Member] | Interest And Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss)		$ 26,714