Shareholders' Equity AOCI Affected Line Item in the Statement of Income (Details) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance		$ (124,938)	$ (119,828)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		177,894	162,313
Income Tax Expense		(63,856)	(60,767)
Interest charges		(31,030)	(29,537)
Purchased gas cost		(311,305)	(240,326)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		7,839	(23,102)
Income Tax Expense		(3,061)	9,006
Income (loss) from continuing operations		4,778	(14,096)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Commodity Contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Purchased gas cost	[1]	7,976	(22,965)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Interest Rate Contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges		$ (137)	$ (137)

[1] Amounts are presented as part of income from discontinued operations on the condensed consolidated statements of income.