Fair Value Measurements - Financial Assets and Financial Liabilities That are Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018
Commodity contracts
Assets:
Derivative assets	$ 944	$ 3,154	$ 2,097
Liabilities:
Derivative liabilities	(3,116)	(146)	(101)
Interest rate contracts
Assets:
Derivative Asset		30
Liabilities:
Derivative liabilities	(4,211)
Level 1 | Commodity contracts
Assets:
Derivative assets	944	3,154	2,097
Liabilities:
Derivative liabilities	(3,116)	(146)	(101)
Level 1 | Interest rate contracts
Assets:
Derivative Asset		0
Liabilities:
Derivative liabilities	0
Level 2 | Commodity contracts
Assets:
Derivative assets	0	0	0
Liabilities:
Derivative liabilities	0	0	0
Level 2 | Interest rate contracts
Assets:
Derivative Asset		30
Liabilities:
Derivative liabilities	(4,211)
Level 3 | Commodity contracts
Assets:
Derivative assets	0	0	0
Liabilities:
Derivative liabilities	0	0	$ 0
Level 3 | Interest rate contracts
Assets:
Derivative Asset		$ 0
Liabilities:
Derivative liabilities	$ 0