Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Commodity Contracts - Fair Value - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	$ 1,807	$ 4,510
Fair value of derivative liabilities, gross	(1,520)	(557)
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	1,735	4,506
Fair value of derivative liabilities, gross	(1,447)	(263)
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of derivative assets, gross	72	4
Fair value of derivative liabilities, gross	(73)	(294)
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