Shareholders' Equity AOCI Affected Line Item in the Statement of Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance expense		$ (128,690)	$ (131,388)	$ (385,867)	$ (379,073)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		111,877	101,835	548,832	487,916
Income Tax Expense		(41,069)	(35,692)	(201,974)	(177,224)
Interest charges		(28,498)	(27,679)	(86,472)	(84,775)
Purchased gas cost		(114,176)	(64,583)	(852,975)	(682,265)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations		(113)	(7,619)	4,511	(29,471)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance expense				(107)	124
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest				(107)	124
Income Tax Expense				39	(45)
Income (loss) from continuing operations				(68)	79
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		(177)	(12,484)	7,526	(48,425)
Income Tax Expense		64	4,865	(2,947)	18,875
Income (loss) from continuing operations				4,579	(29,550)
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]	0	(12,347)	7,976	(48,015)
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		$ (177)	$ (137)	$ (450)	$ (410)

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