Fair Value Measurements - Financial Assets and Financial Liabilities That are Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - Commodity contracts - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Assets:
Derivative assets	$ 2,097	$ 1,807	$ 1,163
Liabilities:
Derivative liabilities	(101)	(1,520)	(1,225)
Level 1
Assets:
Derivative assets	2,097	1,735	1,062
Liabilities:
Derivative liabilities	(101)	(1,447)	(1,157)
Level 2
Assets:
Derivative assets	0	72	101
Liabilities:
Derivative liabilities	0	(73)	(68)
Level 3
Assets:
Derivative assets	0	0	0
Liabilities:
Derivative liabilities	$ 0	$ 0	$ 0