Derivative Instruments (Details 2) (Commodity contracts, USD $) In Thousands	Sep. 30, 2011	Sep. 30, 2010
Asset and liability derivatives
Derivatives designated as hedging instruments	$ 313,537	$ 225,339
Derivatives designated as hedging instruments	108,926	106,721
Derivatives designated as hedging instruments
Asset and liability derivatives
Derivatives designated as hedging instruments	231,946	141,834
Derivatives designated as hedging instruments	11,933	12,097
Derivatives not designated as hedging instruments
Asset and liability derivatives
Derivatives designated as hedging instruments	81,591	83,505
Derivatives designated as hedging instruments	$ 96,993	$ 94,624