Discontinued Operations - Schedule of Cash Flow Hedging Instruments, Statements of Financial Performance and Financial Position, Location (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on discontinuance of cash flow hedging of natural gas marketing commodity contracts reclassified from AOCI	$ 0	$ 10,579
Natural Gas Marketing | Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss reclassified from AOCI for effective portion of natural gas marketing commodity contracts		(2,612)
Gain arising from ineffective portion of natural gas marketing commodity contracts		111
Gain on discontinuance of cash flow hedging of natural gas marketing commodity contracts reclassified from AOCI		10,579
Total impact on purchased gas cost reflected in income from discontinued operations		$ 8,078