Unaudited Interim Financial Information (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Regulatory Asset [Line Items]
Regulatory assets	$ 237,321	$ 263,623
Regulatory asset - future recoverable pension costs	12,100	12,400
Regulatory Liabilities [Line Items]
Regulatory liabilities	517,407	514,725
New Accounting Pronouncement, Early Adoption [Line Items]
Long-term debt	2,314,199	2,188,779
Deferred charges and other assets	317,088	305,019
Regulatory Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	479,667	476,891
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	17,416	20,180
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	13,404	13,404
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6,920	4,250
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	128,947	132,348
Infrastructure Mechanisms [Member]
Regulatory Asset [Line Items]
Regulatory assets	49,098	42,719
Deferred Gas Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	18,345	45,184
Recoverable Loss On Reacquired Debt [Member]
Regulatory Asset [Line Items]
Regulatory assets	13,122	13,761
Deferred Project Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	8,125	7,336
APT Annual Adjustment Mechanism [Member]
Regulatory Asset [Line Items]
Regulatory assets	5,194	7,171
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	1,460	1,539
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets	$ 13,030	$ 13,565