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Unaudited Financial Information - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Sep. 30, 2022
--- | --- | ---
Regulatory Asset [Line Items] |  |
Regulatory assets | $ 2,471,810 | $ 2,605,372
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 1,339,967 | 1,358,907
Regulatory excess deferred taxes |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 506,406 | 545,021
Regulatory cost of removal obligation |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 572,384 | 568,307
Deferred gas costs |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 39,013 | 28,834
Asset retirement obligation |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 5,737 | 5,737
APT annual adjustment mechanism |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 37,443 | 31,138
Pension and postretirement benefit costs |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 151,843 | 156,857
Other |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 27,141 | 23,013
Pension and postretirement benefit costs |  |
Regulatory Asset [Line Items] |  |
Regulatory assets | 26,977 | 31,122
Infrastructure mechanisms |  |
Regulatory Asset [Line Items] |  |
Regulatory assets | 164,818 | 235,972
Winter Storm Uri incremental costs |  |
Regulatory Asset [Line Items] |  |
Regulatory assets | 2,125,787 | 2,109,454
Deferred gas costs |  |
Regulatory Asset [Line Items] |  |
Regulatory assets | 41,096 | 119,742
Regulatory excess deferred taxes |  |
Regulatory Asset [Line Items] |  |
Regulatory assets | 48,033 | 47,311
Recoverable loss on reacquired debt |  |
Regulatory Asset [Line Items] |  |
Regulatory assets | 3,364 | 3,406
Deferred pipeline record collection costs |  |
Regulatory Asset [Line Items] |  |
Regulatory assets | 38,647 | 36,898
Other |  |
Regulatory Asset [Line Items] |  |
Regulatory assets | $ 23,088 | $ 21,467

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