Fair Value Measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Sep. 30, 2012	Mar. 31, 2012
Commodity contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments, assets	$ 4,299	$ 5,468	$ 185
Derivative financial instruments, liabilities	(5,467)	(9,437)	(17,902)
Commodity contracts [Member] | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments, assets	4,299	5,468	185
Derivative financial instruments, liabilities	(406)	(671)	(4,674)
Commodity contracts [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments, assets	0	0	0
Derivative financial instruments, liabilities	(5,061)	(8,766)	(13,228)
Commodity contracts [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments, assets	0	0	0
Derivative financial instruments, liabilities	0	0	0
Interest rate contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments, assets	4,229
Derivative financial instruments, liabilities	(24,298)	(30,522)	(17,345)
Interest rate contracts [Member] | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments, assets	0
Derivative financial instruments, liabilities	0	0	0
Interest rate contracts [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments, assets	4,229
Derivative financial instruments, liabilities	(24,298)	(30,522)	(17,345)
Interest rate contracts [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments, assets	0
Derivative financial instruments, liabilities	$ 0	$ 0	$ 0