Derivatives (Effect of Derivative Instruments Recognized in Income Statements, Derivative Instruments) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives		$ (6,762)	$ 14,731
Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives		(6,762)	14,731
Amount of loss settled during the period		(2,100)	(1,200)
Gain (loss) on derivatives [Member] | Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	[1]	$ (6,762)	$ 14,731

[1] Amounts settled during the 2018 and 2017 periods include net payments of $2.1 million and $1.2 million, respectively.