Unaudited Financial Information - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Sep. 30, 2021
Regulatory Asset [Line Items]
Regulatory assets	$ 2,503,239	$ 2,521,441
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,345,370	1,424,195
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	667,882	705,084
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	547,779	541,511
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	3,988	52,553
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	18,373	18,373
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	31,815	31,110
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	54,786	56,201
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	20,747	19,363
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	41,327	45,922
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	166,560	222,795
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	2,106,840	2,100,728
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	84,014	66,395
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	48,873	45,370
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,655	3,789
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	33,748	32,099
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 18,222	$ 4,343