Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Accumulated Other Comprehensive Income Loss Net Of Tax Abstract
Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax	$ 2,558,000	$ 4,205,000
Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Interest Rate Hedges Effect Net Of Tax	(34,157,000)	(5,468,000)
Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(16,861,000)	(22,109,000)
Accumulated other comprehensive income (loss)	(48,460,000)	(23,372,000)
Other Information Abstract
Other Deferred Cost Amortization Expense	500,000	400,000	400,000
One-time update to to gas delivered estimate			7,600,000
Unrealized gain (loss) on open contracts	(10,400,000)	(7,800,000)	(35,900,000)
Interest Costs Capitalized	1,700,000	3,900,000	4,900,000
Composite depreciation rate for regulated property, plant and equipment	3.60%	3.50%	3.80%
Asset Retirement Obligation	14,000,000	11,400,000
Asset Retirement Costs in Property, Plant and Equipment	5,400,000	3,800,000
Impairment of available-for-sale securities			5,400,000
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	254,666,000	209,564,000
Merger And Integration Costs Net [Member]
Regulatory Asset [Line Items]
Regulatory Assets	6,242,000	6,714,000
Deferred Gas Costs Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets	33,976,000	22,701,000
Regulatory Cost of Removal Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets	8,852,000	31,014,000
Environmental Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	385,000	805,000
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	4,862,000	4,505,000
Deferred Franchise Fees [Member]
Regulatory Asset [Line Items]
Regulatory Assets	379,000	1,161,000
Deferred Miscellaneous Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	3,534,000	1,046,000
Deferred Gas Costs Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	8,130,000	43,333,000
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	464,025,000	381,474,000
Miscellaneous Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 14,025,000	$ 6,112,000

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of interest rate hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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Asset retirement costs recorded as a component of property, plant and equipment that will be depreciated over the remaining life of the underlying associated assets.

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Increase in gross profit associated with a one-time update to our estimate for gas delivered to customers but not yet billed, resulting from base rate changes in several jurisdictions.

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The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

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Accumulated appreciation or loss, net of tax, in value of the total of unsold securities at the end of an accounting period.

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Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's increase (decrease) in deferred hedging gains or losses.

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at fiscal year-end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, and unrealized gains and losses on certain investments in debt and equity securities as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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The amount of interest that was capitalized during the period.

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Amount of other deferred costs charged against earnings in the period.

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Discloses the overall composite depreciation rate for pubic utility plants in service for each year an income statement is presented.

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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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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period

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Amount of unrealized gain (loss) recognized in earnings in the period from the increase (decrease) in fair value of price risk derivatives and nonderivative instruments designated as fair value hedging instruments, as offset by the gain (loss) on the hedged item to the extent that the fair value hedge is determined to be effective.

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