Fair Value Measurements (Recurring Fair Value Measurements) (Details) (Commodity Derivatives [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	$ 55,128	$ 44,019
Liabilities	(3,410)	(10,224)
Financial assets (liabilities)	51,718	33,795
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	46,177	9,698
Liabilities	(2,589)	(9,518)
Financial assets (liabilities)	43,588	180
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	8,951	34,321
Liabilities	(821)	(706)
Financial assets (liabilities)	$ 8,130	$ 33,615