Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	$ 1,697	$ 92
Fair value of derivative liabilities, gross	(2,301)	(6,726)
Commodity Contracts | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	1,697	92
Fair value of derivative liabilities, gross	(2,301)	(6,726)
Commodity Contracts | Fair Value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	679	92
Fair value of derivative liabilities, gross	(2,095)	(2,211)
Commodity Contracts | Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	1,018	0
Fair value of derivative liabilities, gross	(206)	(4,515)
Commodity Contracts | Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	0	0
Fair value of derivative liabilities, gross	$ 0	$ 0