Derivatives (Effect of Derivative Instruments Recognized in Income Statements, Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ (14,461)		$ 8,902		$ (21,223)		$ 23,633
Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	(14,461)		8,902		(21,223)		23,633
Amount of loss settled during the period	(6,900)		(400)		(8,900)		(1,600)
Gain (loss) on derivatives [Member] | Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ (14,461)	[1]	$ 8,902	[1]	$ (21,223)	[2]	$ 23,633	[2]

[1] Amounts settled during the 2018 and 2017 periods include net payments of $6.9 million and $0.4 million, respectively.

[2] Amounts settled during the 2018 and 2017 periods include payments of $8.9 million and $1.6 million, respectively.