Unaudited Financial Information (Details) - USD ($) $ in Thousands	Dec. 31, 2018	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	$ 142,366		$ 143,811
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,512,470		1,510,997
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	740,896		744,895
Regulatory cost of service reserve
Regulatory Liabilities [Line Items]
Regulatory liabilities	19,281		22,508
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	523,644		522,175
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	85,820		94,705
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	12,887		12,887
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	14,354		9,486
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	44,619		35,228
Pension and other postreitrement plans costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	70,969		69,113
Pension and other postreitrement plans costs
Regulatory Asset [Line Items]
Regulatory assets	7,188		6,496
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	85,071		96,739
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	11,621		1,927
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	8,076		8,702
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	22,122		20,467
Rate case costs
Regulatory Asset [Line Items]
Regulatory assets	1,866		2,741
Other
Regulatory Asset [Line Items]
Regulatory assets	6,422		$ 6,739
Other Current Liabilities | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 23,100