Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Commodity Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	$ 4,510	$ 1,307
Fair value of derivative liabilities, gross	(557)	(5,948)
Commodity Contracts | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	4,506	934
Fair value of derivative liabilities, gross	(263)	(4,560)
Commodity Contracts | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	4	373
Fair value of derivative liabilities, gross	(294)	(1,388)
Commodity Contracts | 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
Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative liabilities, gross		(7,016)
Interest Rate Contracts | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative liabilities, gross		0
Interest Rate Contracts | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative liabilities, gross		(7,016)
Interest Rate Contracts | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative liabilities, gross		$ 0