Commodity Derivative Activities - Derivative Table, Fair Value with Netting (Details) - Not Designated as Hedging Instrument [Member] - Commodity Contract [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Total Fair Value, Gross	$ 485	$ 1,081
Total	485	1,081
Prepaid Expenses and Other [Member]
Derivative [Line Items]
Derivative Asset, Gross Amount	1,693	2,035
Derivative Asset, Amount Offset	(374)	(1)
Derivative Asset, As Presented	1,319	2,034
Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Gross Amount	1,638	633
Derivative Asset, Amount Offset	(551)	(67)
Derivative Asset, As Presented	1,087	566
Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Gross Amount	(1,788)	(1,002)
Derivative Liability, Amount Offset	374	0
Derivative Liability, As Presented	(1,414)	(1,002)
Noncurrent Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Gross Amount	(1,058)	(585)
Derivative Liability, Amount Offset	551	68
Derivative Liability, As Presented	$ (507)	$ (517)