Shareholders' Equity AOCI Affected Line Item in the Statement of Income (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance	$ (560,766)	$ (541,868)	$ (505,154)
Interest charges	(115,948)	(116,241)	(129,295)
Purchased gas cost	(1,605,053)	(2,462,119)	(3,358,490)
Income (loss) before income taxes	550,477	510,765	476,819
Income Tax Expense (Benefit)	(200,373)	(195,690)	$ (187,002)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations	(32,262)	(25,450)
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	318	850
Income (loss) before income taxes	318	850
Income Tax Expense (Benefit)	(116)	(310)
Income (loss) from continuing operations	202	540
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) before income taxes	(53,197)	(42,569)
Income Tax Expense (Benefit)	20,733	16,579
Income (loss) from continuing operations	(32,464)	(25,990)
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	(546)	(853)
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	$ (52,651)	$ (41,716)

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=68079674&loc=SL4569616-111683

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a),(b)
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-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
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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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