Derivatives (Effect of Derivative Instruments Recognized in Statement of Operations, Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ (2,614)		$ 6,969		$ 21,019		$ (4,774)
Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	(2,614)		6,969		21,019		(4,774)
Amount of gain (loss) settled during the period	800		(500)		(700)		11,700
Gain (loss) on derivatives [Member] | Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ (2,614)	[1]	$ 6,969	[1]	$ 21,019	[2]	$ (4,774)	[1]

[1] Amounts settled during the 2017 and 2016 periods include net proceeds of $0.8 million and net payments of $0.5 million, respectively.

[2] Amounts settled during the 2017 and 2016 periods include net payments of $0.7 million and net proceeds of $11.7 million, respectively.