Income Taxes (Details) (USD $)	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Current income taxes
Federal	$ 0	$ 0	$ 631,000
State	5,527,000	8,178,000	6,888,000
Deferred income taxes
Federal	169,106,000	124,836,000	103,971,000
State	12,375,000	9,605,000	(13,237,000)
Investment tax credits	(6,000)	(20,000)	(27,000)
Income tax expense	187,002,000	142,599,000	98,226,000
Deferred tax assets
Employee benefit plans	116,157,000	115,970,000
Interest rate agreements	10,565,000	0
Tax net operating loss carryforwards	236,626,000	196,296,000
Tax credit carryforward	21,614,000	20,939,000
Other	28,849,000	38,013,000
Total deferred tax assets	413,811,000	371,218,000
Valuation allowance	(6,969,000)	(1,085,000)
Net deferred tax assets	406,842,000	370,133,000
Deferred tax liabilities
Difference in net book value and net tax value of assets	(1,655,894,000)	(1,445,450,000)
Pension funding	(17,890,000)	(23,480,000)
Gas Cost Adjustments	(31,252,000)	(19,182,000)
Interest rate agreements	0	(21,726,000)
Other	(37,173,000)	(39,045,000)
Total deferred tax liabilities	(1,742,209,000)	(1,548,883,000)
Net deferred tax liabilities	(1,335,367,000)	(1,178,750,000)
Deferred credits for rate regulated entities	(109,000)	(51,000)
Deferred Tax Assets, Operating Loss and Tax Credit Carryforwards [Abstract]
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	10,100,000
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	224,200,000
Deferred Tax Assets, Operating Loss Carryforwards, State and Local	12,400,000
Deferred Tax Assets, Tax Credit Carryforwards, State	1,000,000
Deferred Tax Assets, Charitable Contribution Carryforwards	10,500,000
Valuation Allowance [Abstract]
Valuation allowance	(6,969,000)	(1,085,000)
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	4,158,000	2,817,000
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions	3,846,000	0
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	4,625,000	1,341,000
Unrecognized Tax Benefits, Ending Balance	12,629,000	4,158,000	2,817,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	411,000	0
Unrecognized Tax Benefits, Gross	13,040,000	4,158,000
Unrecognized Tax Benefits, Deferred Federal and State Income Tax Benefits	(4,564,000)	(1,455,000)
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	8,476,000	2,703,000
Other Tax Matters [Abstract]
Other Information Pertaining to Income Taxes	results for fiscal 2012 were favorably impacted by a state tax benefit of $13.6 million (net of federal effects). 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