Fair Value Measurements (Recurring Fair Value Measurements) (Details) (Commodity Derivatives [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	$ 17,176	$ 18,555
Gross Liabilities	(3,273)	(4,513)
Assets	14,782	16,552
Liabilities	(879)	(2,510)
Financial assets (liabilities)	13,903	14,042
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	15,395	18,555
Gross Liabilities	(2,938)	(3,918)
Financial assets (liabilities)	12,457	14,637
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Assets	1,781	0
Gross Liabilities	(335)	(595)
Financial assets (liabilities)	1,446	(595)
Effect of Netting [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Effect of netting on assets	(2,394)	(2,003)
Effect of netting on liabilities	2,394	2,003
Financial assets (liabilities)	$ 0	$ 0