Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Regulatory Assets [Line Items]
Regulatory Assets		$ 192,339	$ 250,309
Regulatory Liabilities [Line Items]
Regulatory Liabilities		524,532	545,759
Regulatory Assets and Liabilities, Other Disclosures [Abstract]
Regulatory Asset - Future Recoverable Pension Costs		16,600	18,800
Property, Plant and Equipment [Line Items]
Interest Costs Capitalized		$ 2,300	$ 1,500	$ 1,900
Composite depreciation rate for regulated property, plant and equipment		3.30%	3.30%	3.30%
Other Information [Abstract]
Unrealized gain (loss) on open contracts		$ (2,400)	$ 9,600	$ 9,000
Asset Retirement Obligation		11,100	10,500
Asset Retirement Costs in Property, Plant and Equipment		4,800	5,900
Cash Held in Margin Accounts		43,500	25,800
Regulatory Cost Of Removal Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		483,676	490,448
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		28,100	35,063
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		9,063	10,508
Deferred Franchise Fees Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		0	5,268
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		3,693	4,472
Pension And Postretirement Benefit Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	[1]	121,183	162,777
Infrastructure Mechanisms [Member]
Regulatory Assets [Line Items]
Regulatory Assets	[2]	32,813	26,948
Deferred Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets		9,715	20,069
Recoverable Loss On Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory Assets		16,319	18,877
APT Annual Adjustment Mechanism Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets		1,002	8,479
Rate Case Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets		1,533	3,757
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets		$ 9,774	$ 9,402
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		43 years

[1] Includes $16.6 million and $18.8 million of pension and postretirement expense deferred pursuant to regulatory authorization.

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