Financial Instruments and Risk Management (Recognized Gains and Losses for Derivative Instruments Not Designated as Hedging Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Not Designated as Hedging Instrument | Commodity derivative contracts | Gain (loss) on crude contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on crude contracts	$ 57,916	$ (37,624)	$ 57,916	$ (67,126)