Segment Information - Schedule of Segment Information (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	Sep. 30, 2014
Segment Reporting Information
Revenues	$ 500,573us-gaap_SalesRevenueNet	$ 513,956us-gaap_SalesRevenueNet	$ 787,879us-gaap_SalesRevenueNet	$ 812,855us-gaap_SalesRevenueNet
Cost of sales	278,336	300,424	421,388	452,289
Depreciation and amortization	15,644	14,643	31,069	29,093
Operating income (loss)	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
Capital expenditures	41,280	31,648	96,309	65,902
Total assets (at period end)	2,506,043us-gaap_Assets	2,340,617us-gaap_Assets	2,506,043us-gaap_Assets	2,340,617us-gaap_Assets	2,354,643us-gaap_Assets
Goodwill (at period end)	182,145us-gaap_Goodwill	182,145us-gaap_Goodwill	182,145us-gaap_Goodwill	182,145us-gaap_Goodwill	182,145us-gaap_Goodwill
Gas Utility
Segment Reporting Information
Revenues	468,000	480,163	728,478	751,725
Cost of sales	258,155	278,748	385,363	414,235
Depreciation and amortization	14,489	13,575	28,769	26,959
Operating income (loss)	139,303	134,560	211,149	216,613
Interest expense	10,104	8,362	20,234	16,748
Income before income taxes	129,199	126,198	190,915	199,865
Capital expenditures	39,202	30,039	92,694	62,851
Total assets (at period end)	2,359,060	2,195,437	2,359,060	2,195,437
Goodwill (at period end)	182,145	182,145	182,145	182,145
Electric Utility
Segment Reporting Information
Revenues	32,323	33,552	58,746	60,513
Cost of sales	20,181	21,676	36,025	38,054
Depreciation and amortization	1,155	1,068	2,300	2,134
Operating income (loss)	3,510	3,492	7,229	7,181
Interest expense	507	444	1,026	855
Income before income taxes	3,003	3,048	6,203	6,326
Capital expenditures	2,078	1,609	3,615	3,051
Total assets (at period end)	146,983	145,180	146,983	145,180
Goodwill (at period end)	0	0	0	0
Other
Segment Reporting Information
Revenues	250	241	655	617
Cost of sales	0	0	0	0
Depreciation and amortization	0	0	0	0
Operating income (loss)	(114)	(98)	(39)	3
Interest expense	0	0	0	0
Income before income taxes	(114)	(98)	(39)	3
Capital expenditures	0	0	0	0
Total assets (at period end)	0	0	0	0
Goodwill (at period end)	$ 0	$ 0	$ 0	$ 0

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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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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The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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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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Interest and debt related expenses associated with nonoperating financing activities of the entity.

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Amount of acquisition of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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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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