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Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Sep. 30, 2020 | Mar. 31, 2020
--- | --- | --- | ---
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | $ 60.4 | $ 69.5 | $ 68.0
Regulatory assets, non current | 1,133.6 | 1,069.4 | 770.7
Total Regulatory Assets | 1,194.0 | 1,138.9 | 838.7
Regulatory liabilities, current | 56.3 | 113.0 | 31.8
Regulatory liabilities, non current | 423.5 | 343.7 | 472.3
Total Regulatory Liabilities | 479.8 | 456.7 | 504.1
Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | 37.5 | 32.1 | 29.4
Regulatory assets, non current | 604.3 | 548.7 | 512.1
Total Regulatory Assets | 641.8 | 580.8 | 541.5
Regulatory liabilities, current | 37.6 | 103.2 | 24.3
Regulatory liabilities, non current | 353.5 | 274.8 | 400.4
Total Regulatory Liabilities | 391.1 | 378.0 | 424.7
Spire Alabama |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | 11.8 | 20.4 | 23.3
Regulatory assets, non current | 501.2 | 489.9 | 230.1
Total Regulatory Assets | 513.0 | 510.3 | 253.4
Regulatory liabilities, current | 14.2 | 3.9 | 2.4
Regulatory liabilities, non current | 18.0 | 18.5 | 21.6
Total Regulatory Liabilities | 32.2 | 22.4 | 24.0
Pension and Other Postretirement Benefits |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | 30.9 | 30.6 | 30.1
Regulatory assets, non current | 418.3 | 439.3 | 415.6
Pension and Other Postretirement Benefits | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | 21.9 | 21.9 | 21.9
Regulatory assets, non current | 324.6 | 332.6 | 337.5
Pension and Other Postretirement Benefits | Spire Alabama |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | 8.0 | 7.7 | 7.2
Regulatory assets, non current | 88.5 | 98.2 | 72.6
Unamortized Purchased Gas Adjustments |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | 0.1 | 5.5 | 10.4
Regulatory assets, non current | 51.5 | 12.1 | 0.0
Unamortized Purchased Gas Adjustments | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, non current | 51.5 | 12.1 | 0.0
Unamortized Purchased Gas Adjustments | Spire Alabama |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current |  | 5.5 | 10.4
Other |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | 29.4 | 33.4 | 27.5
Regulatory assets, non current | 62.5 | 59.3 | 49.6
Other | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | 15.6 | 10.2 | 7.5
Regulatory assets, non current | 45.1 | 42.7 | 30.1
Other | Spire Alabama |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, current | 3.8 | 7.2 | 5.7
Regulatory assets, non current | 1.1 | 0.9 | 3.2
Future Income Taxes Due from Customers |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, non current | 127.4 | 123.5 | 113.5
Future Income Taxes Due from Customers | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, non current | 118.6 | 114.6 | 106.8
Future Income Taxes Due from Customers | Spire Alabama |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, non current | 2.2 | 2.2 | 0.0
Accrued Cost of Removal |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, non current | 430.7 | 395.6 | 154.3
Accrued Cost of Removal | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, non current | 21.3 | 7.1 | 0.0
Accrued Cost of Removal | Spire Alabama |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, non current | 409.4 | 388.6 | 154.3
Energy Efficiency |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, non current | 43.2 | 39.6 | 37.7
Energy Efficiency | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory assets, non current | 43.2 | 39.6 | 37.7
Pension and Other Postretirement Benefits |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, current | 5.8 | 5.8 | 5.8
Regulatory liabilities, non current | 170.2 | 157.6 | 152.5
Pension and Other Postretirement Benefits | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, current | 3.6 | 3.6 | 3.6
Regulatory liabilities, non current | 151.9 | 140.4 | 130.7
Pension and Other Postretirement Benefits | Spire Alabama |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, current | 2.2 | 2.2 | 2.2
Regulatory liabilities, non current | 14.3 | 14.8 | 17.8
Unamortized Purchased Gas Adjustments |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, current | 28.9 | 73.1 | 8.0
Regulatory liabilities, non current | 49.7 | 4.4 | 82.6
Unamortized Purchased Gas Adjustments | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, current | 17.0 | 72.3 | 7.1
Regulatory liabilities, non current | 49.7 | 4.4 | 82.6
Unamortized Purchased Gas Adjustments | Spire Alabama |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, current | 10.6 | 0.0 | 0.0
Other |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, current | 21.6 | 34.1 | 18.0
Regulatory liabilities, non current | 41.7 | 14.3 | 31.3
Other | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, current | 17.0 | 27.3 | 13.6
Regulatory liabilities, non current | 36.1 | 8.6 | 25.5
Other | Spire Alabama |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, current | 1.4 | 1.7 | 0.2
Regulatory liabilities, non current | 3.7 | 3.7 | 3.8
Deferred Taxes Due to Customers |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, non current | 133.2 | 138.8 | 174.2
Deferred Taxes Due to Customers | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, non current | 115.8 | 121.4 | 156.8
Accrued Cost of Removal |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, non current | 28.7 | 28.6 | 31.7
Accrued Cost of Removal | Spire Missouri |  |  |
Regulatory Asset And Liabilities [Line Items] |  |  |
Regulatory liabilities, non current | $ 0.0 | $ 0.0 | $ 4.8

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