Discontinued Operations - Statement of income data (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income tax expense					$ 0	$ 6,841	$ 3,731
Income from discontinued operations					0	10,994	4,562
Gain on sale of discontinued operations, net of tax ($0, $10,215 and $0)	$ 0	$ 0	$ 2,716	$ 0	0	2,716	0
Gain on sale of discontinued operations, tax					0	10,215	0
Net income from discontinued operations					$ 0	13,710	4,562
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	1,005,090
Purchased gas cost						277,554	968,118
Operating expenses						7,874	26,184
Operating income						18,046	10,788
Other nonoperating expense						(211)	(2,495)
Income from discontinued operations before income taxes						17,835	8,293
Income tax expense						6,841	3,731
Income from discontinued operations						10,994	4,562
Gain on sale of discontinued operations, net of tax ($0, $10,215 and $0)						2,716	0
Net income from discontinued operations						$ 13,710	$ 4,562