Condensed Consolidated Statements of Income (unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Revenues	$ 500,573us-gaap_SalesRevenueNet	$ 513,956us-gaap_SalesRevenueNet	$ 787,879us-gaap_SalesRevenueNet	$ 812,855us-gaap_SalesRevenueNet
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	278,336	300,424	421,388	452,289
Operating and administrative expenses	58,917us-gaap_OperatingExpenses	54,306us-gaap_OperatingExpenses	105,465us-gaap_OperatingExpenses	95,451us-gaap_OperatingExpenses
Operating and administrative expenses — related parties	4,138	3,769	6,920	5,612
Taxes other than income taxes	4,803	4,801	8,907	8,980
Depreciation	14,757us-gaap_Depreciation	13,815us-gaap_Depreciation	29,315us-gaap_Depreciation	27,437us-gaap_Depreciation
Amortization	887us-gaap_AdjustmentForAmortization	828us-gaap_AdjustmentForAmortization	1,754us-gaap_AdjustmentForAmortization	1,656us-gaap_AdjustmentForAmortization
Other operating income, net	(3,964)us-gaap_OtherOperatingIncomeExpenseNet	(1,941)us-gaap_OtherOperatingIncomeExpenseNet	(4,209)us-gaap_OtherOperatingIncomeExpenseNet	(2,367)us-gaap_OtherOperatingIncomeExpenseNet
Total costs and expenses	357,874us-gaap_CostsAndExpenses	376,002us-gaap_CostsAndExpenses	569,540us-gaap_CostsAndExpenses	589,058us-gaap_CostsAndExpenses
Operating income	142,699us-gaap_OperatingIncomeLoss	137,954us-gaap_OperatingIncomeLoss	218,339us-gaap_OperatingIncomeLoss	223,797us-gaap_OperatingIncomeLoss
Interest expense	10,611us-gaap_InterestAndDebtExpense	8,806us-gaap_InterestAndDebtExpense	21,260us-gaap_InterestAndDebtExpense	17,603us-gaap_InterestAndDebtExpense
Income before income taxes	132,088	129,148	197,079	206,194
Income taxes	52,499us-gaap_IncomeTaxExpenseBenefit	53,038us-gaap_IncomeTaxExpenseBenefit	78,651us-gaap_IncomeTaxExpenseBenefit	84,798us-gaap_IncomeTaxExpenseBenefit
Net income	$ 79,589us-gaap_NetIncomeLoss	$ 76,110us-gaap_NetIncomeLoss	$ 118,428us-gaap_NetIncomeLoss	$ 121,396us-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 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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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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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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