Income Taxes (Details) (USD $)	12 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Deferred tax assets
Accruals not currently deductible for tax purposes	$ 10,327,000	$ 9,182,000
Customer advances	5,271,000	5,723,000
Nonqualified benefit plans	43,924,000	43,427,000
Postretirement benefits	62,274,000	57,386,000
Treasury lock agreements	20,060,000	3,211,000
Unamortized investment tax credit	120,000	183,000
Tax net operating loss and credit carryforwards	95,293,000	63,621,000
Difference between book and tax on mark to market accounting	8,039,000	2,159,000
Other, net	3,529,000	4,559,000
Total deferred tax assets	248,837,000	189,451,000
Deferred tax liabilities
Difference in net book value and net tax value of assets	1,108,063,000	940,914,000
Pension funding	7,533,000	14,936,000
Gas Cost Adjustments	13,570,000	6,473,000
Cost expensed for tax purposes and capitalized for book purposes	3,039,000	2,330,000
Total deferred tax liabilities	1,132,205,000	964,653,000
Net deferred tax liabilities	(883,368,000)	(775,202,000)
Operating Loss Carryforwards [Line Items]
Liabilities associated with uncertain tax positions	0	6,700,000
Penalty and interest expenses	10,000	500,000	100,000
Deferred credits for rate regulated entities	325,000	587,000
Current income taxes
Federal	(11,204,000)	(72,234,000)	(37,141,000)
State	6,533,000	6,179,000	8,720,000
Deferred income taxes
Federal	112,612,000	179,271,000	134,912,000
State	5,920,000	11,429,000	(8,739,000)
Investment tax credits	(172,000)	(283,000)	(390,000)
Income tax expense (benefit)	113,689,000	124,362,000	97,362,000
Federal Alternative Minimum Tax Credit Carryforwards	10,100,000
Internal Revenue Service IRS [Member]
Operating Loss Carryforwards [Line Items]
Amount	75,200,000
Expiration Dates	2029000000
State And Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Amount	$ 9,900,000
Expiration Dates	between 2016 and 2029