Discontinued Operations - Impact of our discontinued operation commodity contracts (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Discontinued Operations and Disposal Groups [Abstract]
Commodity contracts	$ (9,567)	$ 3,516
Fair value adjustment for natural gas inventory designated as the hedged item	12,858	18,079
Total decrease in purchased gas cost reflected in income from discontinued operations	3,291	21,595
Basis ineffectiveness	(597)	(1,390)
Timing ineffectiveness	$ 3,888	$ 22,985