Derivative Instruments (Fair Values of Derivative Instruments in Consolidated Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Accrued Liabilities
Derivatives, Fair Value [Line Items]
Total liability derivatives	$ 2,892	$ 4,235
Accounts Receivable
Derivatives, Fair Value [Line Items]
Total asset derivatives	9,843	2,437
Commodity Forward Contracts | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivatives designated as hedging instruments, Liability Derivatives	1,019	3,262
Derivatives not designated as hedging instruments, Liability Derivatives	1,873	973
Commodity Forward Contracts | Accounts Receivable
Derivatives, Fair Value [Line Items]
Derivatives designated as hedging instruments, Asset Derivatives	8,630	0
Derivatives not designated as hedging instruments, Asset Derivatives	$ 1,213	$ 2,437