Fair Value Measurement (Details) (Commodity Contract [Member], Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Assets:
Derivative Assets	$ 3,108	$ 2,089	$ 27
Liabilities:
Derivative liabilities	(28,290)	(42,598)	(16,538)
Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member]
Assets:
Derivative Assets
Liabilities:
Derivative liabilities
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Derivative Assets	3,108	2,089	27
Liabilities:
Derivative liabilities	(28,290)	(42,598)	(16,538)
Unobservable Inputs (Level 3) [Member]
Assets:
Derivative Assets
Liabilities:
Derivative liabilities