Fair Value Measurements (Recurring Fair Value Measurements) (Details) (Commodity Derivatives [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	$ 25,858	$ 44,019
Liabilities	(11,458)	(10,224)
Financial assets (liabilities)	14,400	33,795
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	21,851	9,698
Liabilities	(9,054)	(9,518)
Financial assets (liabilities)	12,797	180
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	4,007	34,321
Liabilities	(2,404)	(706)
Financial assets (liabilities)	$ 1,603	$ 33,615