Condensed Consolidated Statements of Income (unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Revenues	$ 287,306us-gaap_SalesRevenueNet	$ 298,899us-gaap_SalesRevenueNet
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	143,052	151,865
Operating and administrative expenses	46,548us-gaap_OperatingExpenses	41,145us-gaap_OperatingExpenses
Operating and administrative expenses — related parties	2,782	1,843
Taxes other than income taxes	4,104	4,179
Depreciation	14,558us-gaap_Depreciation	13,622us-gaap_Depreciation
Amortization	867us-gaap_AdjustmentForAmortization	828us-gaap_AdjustmentForAmortization
Other operating income, net	(245)us-gaap_OtherOperatingIncomeExpenseNet	(426)us-gaap_OtherOperatingIncomeExpenseNet
Total costs and expenses	211,666us-gaap_CostsAndExpenses	213,056us-gaap_CostsAndExpenses
Operating income	75,640us-gaap_OperatingIncomeLoss	85,843us-gaap_OperatingIncomeLoss
Interest expense	10,649us-gaap_InterestAndDebtExpense	8,797us-gaap_InterestAndDebtExpense
Income before income taxes	64,991	77,046
Income taxes	26,152us-gaap_IncomeTaxExpenseBenefit	31,760us-gaap_IncomeTaxExpenseBenefit
Net income	$ 38,839us-gaap_NetIncomeLoss	$ 45,286us-gaap_NetIncomeLoss

The aggregate amount of recurring noncash expense charged against earnings in the period to allocate the cost of assets over their estimated remaining economic lives.

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The aggregate cost of goods produced and sold during the reporting period, excluding depreciation, depletion, and amortization.

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Total costs of sales and operating expenses for the period.

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The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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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 from ongoing operations, after income or loss from equity method investments, but before income taxes, 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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Interest and debt related expenses associated with nonoperating financing activities of the entity.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.

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The net result for the period of deducting operating expenses from operating revenues.

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The net amount of other operating income and expenses, the components of which are not separately disclosed on the income statement, from items that are associated with the entity's normal revenue producing operations.

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Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

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All taxes not related to income of the entity or excise or sales taxes levied on the revenue of the entity that are not reported elsewhere. These taxes could include production, real estate, personal property, and pump tax.

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