Unaudited Financial Information (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Oct. 01, 2018	Sep. 30, 2018
Accounting Policies [Abstract]
Cummulative effect on accounting change, on equity or net assets		$ 8,200
Regulatory Asset [Line Items]
Regulatory assets	$ 164,670		$ 143,811
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,489,875		1,510,997
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	731,837		744,895
Regulatory cost of service reserve
Regulatory Liabilities [Line Items]
Regulatory liabilities	6,079		22,508
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	526,403		522,175
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	66,171		94,705
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	12,887		12,887
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	3,038		9,486
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	63,130		35,228
Pension and other postreitrement plans costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	80,330		69,113
Pension and other postreitrement plans costs
Regulatory Asset [Line Items]
Regulatory assets	8,007		6,496
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	111,211		96,739
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	7,227		1,927
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	7,000		8,702
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	25,347		20,467
Rate case costs
Regulatory Asset [Line Items]
Regulatory assets	1,413		2,741
Other
Regulatory Asset [Line Items]
Regulatory assets	4,465		6,739
Other Current Liabilities | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 21,900		$ 5,200