Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Asset	$ 3,464	$ 12,870
Derivative Liability	(475)	(293)
Financial assets (liabilities)	3,214	12,771
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets (liabilities)	225	194
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets (liabilities)	(91)	1,947
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets (liabilities)	3,080	10,630
Effect of Netting [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets (liabilities)	0	0
Commodity Derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Asset	3,464	12,870
Derivative Liability	(475)	(293)
Total commodity derivatives	2,989	12,577
Commodity Derivatives [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	0	0
Gross Liabilities	0	0
Total commodity derivatives	0	0
Commodity Derivatives [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	1,727	3,225
Gross Liabilities	(1,818)	(1,278)
Total commodity derivatives	(91)	1,947
Commodity Derivatives [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	3,098	10,964
Gross Liabilities	(18)	(334)
Total commodity derivatives	3,080	10,630
Commodity Derivatives [Member] \| Effect of Netting [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Effect of netting on assets	(1,361)	(1,319)
Effect of netting on liabilities	1,361	1,319
Total commodity derivatives	0	0
Available-for-sale Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	225	194
Available-for-sale Securities [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	225	194
Available-for-sale Securities [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	0	0
Available-for-sale Securities [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	0	0
Available-for-sale Securities [Member] \| Effect of Netting [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	$ 0	$ 0

Amount of equity securities categorized neither as held-to-maturity nor as trading.

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