Derivative Financial Instruments (Details 2) (USD $) In Thousands	Dec. 31, 2010 Current Assets Derivatives designated as hedging instruments	Jun. 30, 2011 Current Assets Derivatives not designated as hedging instruments	Dec. 31, 2010 Noncurrent Assets Derivatives designated as hedging instruments	Jun. 30, 2011 Noncurrent Assets Derivatives not designated as hedging instruments	Dec. 31, 2010 Current Liabilities Derivatives designated as hedging instruments	Jun. 30, 2011 Current Liabilities Derivatives not designated as hedging instruments	Dec. 31, 2010 Noncurrent liabilities Derivatives designated as hedging instruments	Jun. 30, 2011 Noncurrent liabilities Derivatives not designated as hedging instruments	Dec. 31, 2010 Derivatives designated as hedging instruments
Fair value of derivative assets and liabilities
Fair value: Derivative Assets, Commodity Contracts	$ 43,491	$ 28,985	$ 18,841	$ 10,624
Fair value: Derivative Assets									62,332
Fair value: Derivative Liabilities, Commodity Contracts					(82,044)	(70,100)	(32,557)	(38,233)
Fair value: Derivative Liabilities									$ (114,601)