Unaudited Financial Information (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 158,649		$ 143,811
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,539,805		1,510,997
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	736,634		744,895
Regulatory cost of service reserve
Regulatory Liabilities [Line Items]
Regulatory liabilities	6,175		22,508
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	524,067		522,175
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	124,248		94,705
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	12,887		12,887
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	16,942		9,486
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	48,524		35,228
Pension and other postreitrement plans costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	70,328		69,113
Pension and other postreitrement plans costs
Regulatory Asset [Line Items]
Regulatory assets	7,843		6,496
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	107,649		96,739
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	3,490		1,927
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	7,450		8,702
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	23,914		20,467
Rate case costs
Regulatory Asset [Line Items]
Regulatory assets	1,612		2,741
Other
Regulatory Asset [Line Items]
Regulatory assets	6,691		$ 6,739
Other Current Liabilities | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 24,000