Derivatives (Effect of Derivative Instruments Recognized in Income Statements, Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ 4,237	$ (4,385)	$ 5,232	$ (25,608)
Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	4,237	(4,385)	5,232	(25,608)
Amount of gains (loss) settled during the period	6,515	(9,112)	11,829	(18,040)
Gain (loss) on derivatives [Member] | Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ 4,237	$ (4,385)	$ 5,232	$ (25,608)