Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2011
Regulatory Assets [Line Items]
Regulatory Assets	$ 286,748,000		$ 392,263,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities	453,581,000		488,397,000
Regulatory Assets and Liabilities, Other Disclosures [Abstract]
Regulatory Asset - Future Recoverable Pension Costs	17,400,000		7,600,000
Regulatory Noncurrent Asset, Amortization Period	20 years
Amortization of Regulatory Asset	500,000		500,000		500,000
Property, Plant and Equipment [Line Items]
Interest Costs Capitalized	1,900,000		2,600,000		1,700,000
Composite depreciation rate for regulated property, plant and equipment	3.33%		3.60%		3.60%
Other Information [Abstract]
Unrealized gain (loss) on open contracts	9,000,000		(8,000,000)		(10,400,000)
Asset Retirement Obligation	6,800,000		10,500,000
Asset Retirement Costs in Property, Plant and Equipment	3,300,000		5,800,000
Asset Impairment Charges Park City			5,300,000		11,000,000
Asset Impairment Charges Fort Necessity					19,300,000
Finite-Lived Intangible Asset, Useful Life	10 years
Cash Collateral Right To Reclaim Cash	24,800,000		23,700,000
Supplemental Employee Retirement Plan, Defined Benefit [Member] | Fair Value Inputs Level 3 [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Investments, Fair Value Disclosure	200,000
Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives for nonregulated property, plant and equipment	3 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives for nonregulated property, plant and equipment	50 years
Deferred Gas Costs Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	16,481,000		23,072,000
Deferred Franchise Fees Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,689,000		0
Regulatory Cost Of Removal Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	427,524,000		459,688,000
Miscellaneous Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	7,887,000		5,637,000
Pension And Postretirement Benefit Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	187,977,000	[1]	296,160,000	[1]
Merger And Integration Costs Net [Member]
Regulatory Assets [Line Items]
Regulatory Assets	5,250,000		5,754,000
Deferred Gas Costs Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	15,152,000		31,359,000
Regulatory Cost of Removal Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	10,008,000		10,500,000
Rate Case Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	6,329,000		4,661,000
Deferred Franchise Fees [Member]
Regulatory Assets [Line Items]
Regulatory Assets	0		2,714,000
Texas 8.209 [Member]
Regulatory Assets [Line Items]
Regulatory Assets	30,364,000	[2]	5,370,000	[2]
APT Annual Adjustment Mechanism Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	5,853,000		4,539,000
Recoverable Loss On Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory Assets	21,435,000		23,944,000
Deferred Miscellaneous Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 4,380,000		$ 7,262,000

[1] Includes $17.4 million and $7.6 million of pension and postretirement expense deferred pursuant to regulatory authorization.

[2] Texas Rule 8.209 is a Railroad Commission rule that allows for the deferral of all expenses associated with capital expenditures incurred pursuant to this rule, including the recording of interest on the deferred expenses until the next rate proceeding (rate case or annual rate filing), at which time investment and costs would be recovered through base rates.