Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Regulatory Assets [Line Items]
Regulatory Assets	$ 176,798	$ 263,623
Regulatory Liabilities [Line Items]
Regulatory Liabilities	555,657	514,725
Regulatory Assets and Liabilities, Other Disclosures [Abstract]
Regulatory Asset - Future Recoverable Pension Costs	9,400	12,400
Property, Plant and Equipment [Abstract]
Interest Costs Capitalized	$ 2,500	$ 2,800	$ 2,300
Composite depreciation rate for regulated property, plant and equipment	3.10%	3.20%	3.30%
Derivative [Line Items]
Cash Held in Margin Accounts		$ 50,400
Asset Retirement Costs in Property, Plant and Equipment	$ 7,800	8,100
Asset Retirement Obligation	12,800	13,400
Distribution Segment [Member]
Derivative [Line Items]
Cash Held in Margin Accounts	0	25,700
Regulatory Cost Of Removal Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	521,330	476,891
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	15,559	20,180
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	12,827	13,404
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	5,941	4,250
Pension And Postretirement Benefit Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	26,826	132,348
Infrastructure Mechanisms [Member]
Regulatory Assets [Line Items]
Regulatory Assets	46,437	42,719
Deferred Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	65,714	45,184
Recoverable Loss On Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory Assets	11,208	13,761
Deferred Pipeline Record Collection Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	11,692	7,336
APT Annual Adjustment Mechanism Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	2,160	7,171
Rate Case Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	2,629	1,539
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 10,132	$ 13,565