Income Taxes (Details) (USD $)	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Deferred tax assets
Accruals not currently deductible for tax purposes	$ 7,906,000	$ 10,327,000
Customer advances	4,721,000	5,271,000
Nonqualified benefit plans	48,513,000	43,924,000
Postretirement benefits	62,802,000	62,274,000
Treasury lock agreements	25,448,000	20,060,000
Unamortized investment tax credit	14,000	120,000
Tax net operating loss and credit carryforwards	164,419,000	95,293,000
Difference between book and tax on mark to market accounting	2,342,000	8,039,000
Other, net	7,223,000	3,529,000
Total deferred tax assets	323,388,000	248,837,000
Deferred tax liabilities
Difference in net book value and net tax value of assets	1,254,698,000	1,108,063,000
Pension funding	32,812,000	7,533,000
Gas Cost Adjustments	21,806,000	13,570,000
Cost expensed for tax purposes and capitalized for book purposes	2,065,000	3,039,000
Total deferred tax liabilities	1,311,381,000	1,132,205,000
Net deferred tax liabilities	(987,993,000)	(883,368,000)
Operating Loss Carryforwards [Line Items]
Liabilities associated with uncertain tax positions	1,800,000	6,700,000
Penalty and interest expenses	10,000	10,000	500,000
Deferred credits for rate regulated entities	140,000	325,000
Current income taxes
Federal	631,000	(13,298,000)	(70,884,000)
State	6,888,000	6,841,000	6,849,000
Deferred income taxes
Federal	103,971,000	107,950,000	172,690,000
State	(13,237,000)	5,498,000	10,831,000
Investment tax credits	(27,000)	(172,000)	(283,000)
Income tax expense (benefit)	98,226,000	106,819,000	119,203,000
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.
Internal Revenue Service IRS [Member]
Operating Loss Carryforwards [Line Items]
Amount	143,200,000
Expiration Dates	2029
State And Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Amount	10,600,000
Expiration Dates	between 2016 and 2030
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	$ 500,000
Expiration dates	2018