Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	$ 6,763	$ 18,555
Gross Liabilities	(18,455)	(4,513)
Financial assets (liabilities)	(11,692)	14,042
Commodity Derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Effect of netting on assets	(3,546)	(2,003)
Effect of netting on liabilities	3,546	2,003
Assets	3,217	16,552
Liabilities	(14,909)	(2,510)
Commodity Derivatives [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	6,727	18,555
Gross Liabilities	(15,883)	(3,918)
Financial assets (liabilities)	(9,156)	14,637
Commodity Derivatives [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	36	0
Gross Liabilities	(2,572)	(595)
Financial assets (liabilities)	(2,536)	(595)
Commodity Derivatives [Member] | Effect of Netting [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets (liabilities)	$ 0	$ 0