Accumulated Other Comprehensive Income Affected Line Item in the Statement of Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance	$ (133,460)	$ (124,675)	$ (252,042)	$ (240,432)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	221,202	216,963	377,456	353,436
Income Tax Expense	(83,518)us-gaap_IncomeTaxExpenseBenefit	(83,596)us-gaap_IncomeTaxExpenseBenefit	(142,177)us-gaap_IncomeTaxExpenseBenefit	(133,053)us-gaap_IncomeTaxExpenseBenefit
Interest charges	(27,447)us-gaap_InterestExpense	(31,601)us-gaap_InterestExpense	(57,211)us-gaap_InterestExpense	(63,716)us-gaap_InterestExpense
Purchased gas cost	(1,019,330)us-gaap_CostOfNaturalGasPurchases	(1,468,577)us-gaap_CostOfNaturalGasPurchases	(1,854,810)us-gaap_CostOfNaturalGasPurchases	(2,323,478)us-gaap_CostOfNaturalGasPurchases
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations	(8,064)	3,938	(8,132)	1,674
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	0	358	6	358
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	0	358	6	358
Income Tax Expense	0	(131)	(2)	(131)
Income (loss) from continuing operations	0	227	4	227
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Accumulated Net Gain (Loss) from Interest Rate Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(136)	(1,057)	(580)	(2,115)
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Accumulated Net Gain (Loss) from Commodity Contracts Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Purchased gas cost	(13,078)	7,184	(12,734)	4,574
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [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	(13,214)	6,127	(13,314)	2,459
Income Tax Expense	5,150	(2,416)	5,178	(1,012)
Income (loss) from continuing operations	$ (8,064)	$ 3,711	$ (8,136)	$ 1,447

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

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items, and 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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-Name Accounting Standards Codification

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Other generally recurring costs associated with normal operations excluding those directly related to the marketing or selling of products and services not otherwise defined.

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-Name Accounting Standards Codification

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