Discontinued Operations - Disposal Groups, Including Discontinued Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income tax expense			$ 0	$ 6,841
Income from discontinued operations			0	10,994
Gain on sale from discontinued operations, net of tax ($10,215)			0	2,716
Net income from discontinued operations	$ 0	$ 0	0	13,710
Gain on sale of discontinued operations, tax			$ 0	10,215
Natural Gas Marketing | Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues				303,474
Purchased gas cost				277,554
Operating expenses				7,874
Operating income				18,046
Other nonoperating expense				(211)
Income from discontinued operations before income taxes				17,835
Income tax expense				6,841
Income from discontinued operations				10,994
Gain on sale from discontinued operations, net of tax ($10,215)				2,716
Net income from discontinued operations				13,710
Depreciation and amortization expense				185
Capital expenditures				0
Non-cash loss in commodity contract cash flow hedges				$ (8,165)