Unaudited Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Sep. 30, 2019
Regulatory Asset [Line Items]
Regulatory assets	$ 242,058	$ 284,547
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,395,017	1,386,126
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	721,049	726,307
Regulatory excess deferred taxes | Other Current Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 21,700	21,200
Regulatory excess deferred taxes | Other Current Liabilities | Minimum
Regulatory Liabilities [Line Items]
Return basis, term	15 years
Regulatory excess deferred taxes | Other Current Liabilities | Maximum
Regulatory Liabilities [Line Items]
Return basis, term	46 years
Regulatory cost of service reserve
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 4,747	5,238
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	519,538	528,893
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	42,142	14,112
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	17,054	17,054
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	72,732	78,402
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	17,755	16,120
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	83,783	86,089
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	108,997	131,894
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	10,386	23,766
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	6,102	6,551
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	27,414	26,418
Rate case costs
Regulatory Asset [Line Items]
Regulatory assets	1,052	1,346
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 4,324	$ 8,483