Fair Value Measurements - Financial Assets and Financial Liabilities That are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Assets:
Derivative assets	$ 806		$ 177
Liabilities:
Derivative liabilities	(10,923)		(9,955)
Fair Value, Measurements, Recurring | Commodity Contracts
Assets:
Derivative assets	234	$ 1,307	177
Liabilities:
Derivative liabilities	(6,543)	(5,948)	(9,955)
Fair Value, Measurements, Recurring | Interest rate contracts
Assets:
Derivative assets	572
Liabilities:
Derivative liabilities	(4,380)	(7,016)
Fair Value, Measurements, Recurring | Level 1 | Commodity Contracts
Assets:
Derivative assets	234	934	0
Liabilities:
Derivative liabilities	(4,986)	(4,560)	(7,355)
Fair Value, Measurements, Recurring | Level 1 | Interest rate contracts
Assets:
Derivative assets	0
Liabilities:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Level 2 | Commodity Contracts
Assets:
Derivative assets	0	373	177
Liabilities:
Derivative liabilities	(1,557)	(1,388)	(2,600)
Fair Value, Measurements, Recurring | Level 2 | Interest rate contracts
Assets:
Derivative assets	572
Liabilities:
Derivative liabilities	(4,380)	(7,016)
Fair Value, Measurements, Recurring | Level 3 | Commodity Contracts
Assets:
Derivative assets	0	0	0
Liabilities:
Derivative liabilities	0	0	$ 0
Fair Value, Measurements, Recurring | Level 3 | Interest rate contracts
Assets:
Derivative assets	0
Liabilities:
Derivative liabilities	$ 0	$ 0