Derivatives (Effect of Derivative Instruments Recognized in Income Statements, Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ (4,385)		$ (2,614)		$ (25,608)		$ 21,019
Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	(4,385)		(2,614)		(25,608)		21,019
Amount of gains (loss) settled during the period	(9,100)		800		(18,000)		(700)
Gain (loss) on derivatives [Member] | Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ (4,385)	[1]	$ (2,614)	[1]	$ (25,608)	[2]	$ 21,019	[2]

[1] Amounts settled during the 2018 and 2017 periods include net payments of $9.1 million and net proceeds of $0.8 million, respectively.

[2] Amounts settled during the 2018 and 2017 periods include net payments of $18.0 million and $0.7 million, respectively.