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	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017
Assets:
Derivative assets	$ 1,051	$ 1,807	$ 2,157
Liabilities:
Derivative liabilities	(589)	(1,520)	(152)
Level 1
Assets:
Derivative assets	1,042	1,735	2,157
Liabilities:
Derivative liabilities	(582)	(1,447)	(133)
Level 2
Assets:
Derivative assets	9	72	0
Liabilities:
Derivative liabilities	(7)	(73)	(19)
Level 3
Assets:
Derivative assets	0	0	0
Liabilities:
Derivative liabilities	$ 0	$ 0	$ 0