Regulation - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Regulatory Assets [Line Items]
Regulatory assets	$ 554,928	$ 2,605,372
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,284,334	1,353,170
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	384,513	545,021
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	582,867	568,307
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	23,093	28,834
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	49,894	31,138
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	215,913	156,857
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	28,054	23,013
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets	20,629	31,122
Infrastructure mechanisms
Regulatory Assets [Line Items]
Regulatory assets	229,996	235,972
Winter Storm Uri incremental costs
Regulatory Assets [Line Items]
Regulatory assets	32,115	2,109,454
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory assets	148,297	119,742
Regulatory excess deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	47,549	47,311
Recoverable loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory assets	3,238	3,406
Deferred pipeline record collection costs
Regulatory Assets [Line Items]
Regulatory assets	54,008	36,898
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 19,096	$ 21,467