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	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Assets:
Derivative assets	$ 697	$ 1,807	$ 7,077
Liabilities:
Derivative liabilities	(2,263)	(1,520)	(295)
Level 1
Assets:
Derivative assets	678	1,735	7,077
Liabilities:
Derivative liabilities	(2,151)	(1,447)	0
Level 2
Assets:
Derivative assets	19	72	0
Liabilities:
Derivative liabilities	(112)	(73)	(295)
Level 3
Assets:
Derivative assets	0	0	0
Liabilities:
Derivative liabilities	$ 0	$ 0	$ 0