Fair Value Measurements (Recurring Fair Value Measurements) (Details) - Commodity Derivatives [Member] - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Asset	$ 11,900	$ 31,139
Derivative Liability	0	0
Financial assets (liabilities)	11,900	31,139
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	9,418	27,784
Gross Liabilities	0	0
Financial assets (liabilities)	9,418	27,784
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	2,482	3,355
Gross Liabilities	0	0
Financial assets (liabilities)	$ 2,482	$ 3,355