Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets (liabilities)	$ 14,000
Commodity Derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	16,552	36,452
Liabilities	(2,510)	(2,657)
Financial assets (liabilities)	14,042	33,795
Commodity Derivatives [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	18,555	9,698
Liabilities	(3,918)	(9,518)
Financial assets (liabilities)	14,637	180
Commodity Derivatives [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	0	34,321
Liabilities	(595)	(706)
Financial assets (liabilities)	(595)	33,615
Commodity Derivatives [Member] | Effect of Netting [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	(2,003)	(7,567)
Liabilities	2,003	7,567
Financial assets (liabilities)	$ 0	$ 0