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, 2016	Sep. 30, 2015	Jun. 30, 2015
Commodity contracts
Assets:
Derivative assets	$ 5,718	$ 1,307	$ 1,979
Liabilities:
Derivative liabilities	(732)	(5,948)	(4,995)
Interest rate contracts
Liabilities:
Derivative liabilities		(7,016)
Level 1 | Commodity contracts
Assets:
Derivative assets	5,715	934	1,510
Liabilities:
Derivative liabilities	(341)	(4,560)	(2,390)
Level 1 | Interest rate contracts
Liabilities:
Derivative liabilities		0
Level 2 | Commodity contracts
Assets:
Derivative assets	3	373	469
Liabilities:
Derivative liabilities	(391)	(1,388)	(2,605)
Level 2 | Interest rate contracts
Liabilities:
Derivative liabilities		(7,016)
Level 3 | Commodity contracts
Assets:
Derivative assets	0	0	0
Liabilities:
Derivative liabilities	$ 0	0	$ 0
Level 3 | Interest rate contracts
Liabilities:
Derivative liabilities		$ 0