Cost of natural gas purchased and associated transportation and storage costs during the period.

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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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Amount after tax of income (loss) from continuing operations including portion attributable to the noncontrolling interest.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the cost of borrowed funds accounted for as interest expense.

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Amount of selling, general and administrative expense classified as other.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Shareholders' Equity AOCI Affected Line Item in the Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance									$ (546,798)	$ (538,592)	$ (516,406)
Interest charges									(120,182)	(114,812)	(116,241)
Purchased gas cost	$ (72,561)	$ (114,176)	$ (427,494)	$ (311,305)	$ (63,927)	$ (64,583)	$ (377,356)	$ (240,326)	(925,536)	(746,192)	(1,295,675)
Income from continuing operations before income taxes									604,094	542,184	495,172
Income Tax Expense (Benefit)									(221,383)	(196,642)	$ (189,549)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations									4,141	(32,262)
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									(97)	318
Income from continuing operations before income taxes									(97)	318
Income Tax Expense (Benefit)									35	(116)
Income (loss) from continuing operations									(62)	202
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 from continuing operations before income taxes									6,924	(53,197)
Income Tax Expense (Benefit)									(2,721)	20,733
Income (loss) from continuing operations									4,203	(32,464)
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									(1,043)	(546)
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									$ 7,967	$ (52,651)