Derivative Instruments and Hedging Activities (Derivatives Designated as Hedging Instruments) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Asset derivatives designated as hedging instruments	$ 7,531	$ 5,324
Liability derivatives designated as hedging instruments	17,837	31,634
Other Current Assets [Member] | Oil Contract [Member]
Asset derivatives designated as hedging instruments	3,381	0
Other Current Assets [Member] | Natural Gas Contracts [Member]
Asset derivatives designated as hedging instruments	2,302	5,324
Other Assets, Net [Member] | Oil Contract [Member]
Asset derivatives designated as hedging instruments	1,816	0
Other Assets, Net [Member] | Natural Gas Contracts [Member]
Asset derivatives designated as hedging instruments	32	0
Other Assets, Net [Member] | Interest Rate Swap [Member]
Asset derivatives designated as hedging instruments	0	0
Other Liabilities [Member] | Oil Contract [Member]
Liability derivatives designated as hedging instruments	399	0
Other Liabilities [Member] | Natural Gas Contracts [Member]
Liability derivatives designated as hedging instruments	226	913
Other Liabilities [Member] | Interest Rate Swap [Member]
Liability derivatives designated as hedging instruments	0	115
Accrued Liabilities [Member] | Oil Contract [Member]
Liability derivatives designated as hedging instruments	16,055	28,855
Accrued Liabilities [Member] | Interest Rate Swap [Member]
Liability derivatives designated as hedging instruments	$ 1,157	$ 1,751