Income Taxes (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Current income taxes
Federal		$ 0	$ 631,000	$ (13,298,000)
State		8,178,000	6,888,000	6,841,000
Deferred income taxes
Federal		124,836,000	103,971,000	107,950,000
State		9,605,000	(13,237,000)	5,498,000
Investment tax credits		(20,000)	(27,000)	(172,000)
Income tax expense		142,599,000	98,226,000	106,819,000
Deferred tax assets
Accruals not currently deductible for tax purposes		11,496,000	7,906,000
Customer advances		4,279,000	4,721,000
Nonqualified benefit plans		52,051,000	48,513,000
Postretirement benefits		63,919,000	62,802,000
Treasury lock agreements		0	25,448,000
Unamortized investment tax credit		6,000	14,000
Tax net operating loss and credit carryforwards		206,996,000	164,419,000
Difference between book and tax on mark to market accounting		2,271,000	2,342,000
Other, net		0	7,223,000
Total deferred tax assets		341,018,000	323,388,000
Deferred tax liabilities
Difference in net book value and net tax value of assets		(1,445,450,000)	(1,254,698,000)
Pension funding		(23,480,000)	(32,812,000)
Gas Cost Adjustments		(19,182,000)	(21,806,000)
Interest rate agreements		(21,726,000)	0
Cost expensed for tax purposes and capitalized for book purposes		(2,815,000)	(2,065,000)
Other, net		(7,115,000)	0
Total deferred tax liabilities		(1,519,768,000)	(1,311,381,000)
Net deferred tax liabilities		(1,178,750,000)	(987,993,000)
Operating Loss Carryforwards [Line Items]
Liabilities associated with uncertain tax positions		3,200,000			6,700,000
Liability for Uncertain Tax Positions, Decrease due to Settlement	6,700,000
Settlement of uncertain tax positions				5,000,000
Deferred credits for rate regulated entities		(51,000)	140,000
Other Information Pertaining to Income Taxes		results for fiscal 2012 were favorably impacted by a state tax benefit of $13.6 million. Due to the completion of the sale of our Missouri, Iowa and Illinois service areas in the fiscal fourth quarter, the Company updated its analysis of the tax rate at which deferred taxes would reverse in the future to reflect the sale of these service areas. The updated analysis supported a reduction in the deferred tax rate which when applied to the balance of taxable income deferred to future periods resulted in a reduction of the Company’s overall deferred tax liability.
State tax benefit			13,600,000
Deferred Tax Assets, Valuation Allowance		1,100,000
Internal Revenue Service IRS [Member]
Operating Loss Carryforwards [Line Items]
Amount		185,300,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Amount		11,000,000
Federal Alternative Minimum Tax Credit Carryforward [Member]
Other Tax Carryforward [Line Items]
Amount		10,100,000
State Tax Credit Carryforward [Member]
Other Tax Carryforward [Line Items]
Amount		$ 600,000