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Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Sep. 30, 2019
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Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | $ 69.5 | $ 78.6
Regulatory assets, non current | 1,069.4 | 767.6
Total Regulatory Assets | 1,138.9 | 846.2
Regulatory liabilities, current | 113.0 | 60.8
Regulatory liabilities, non current | 343.7 | 399.0
Total Regulatory Liabilities | 456.7 | 459.8
Pension and Other Postretirement Benefits |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 30.6 | 30.1
Regulatory assets, non current | 439.3 | 416.6
Unamortized Purchased Gas Adjustments |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 5.5 | 18.2
Regulatory assets, non current | 12.1 | 9.1
Other |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 33.4 | 30.3
Regulatory assets, non current | 59.3 | 45.0
Future Income Taxes Due from Customers |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, non current | 123.5 | 111.0
Accrued Cost of Removal |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, non current | 395.6 | 150.9
Energy Efficiency |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, non current | 39.6 | 35.0
Spire Missouri |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 32.1 | 29.4
Regulatory assets, non current | 548.7 | 507.5
Total Regulatory Assets | 580.8 | 536.9
Regulatory liabilities, current | 103.2 | 52.3
Regulatory liabilities, non current | 274.8 | 326.5
Total Regulatory Liabilities | 378.0 | 378.8
Spire Missouri | Pension and Other Postretirement Benefits |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 21.9 | 21.9
Regulatory assets, non current | 332.6 | 333.3
Spire Missouri | Unamortized Purchased Gas Adjustments |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 0.0 | 0.0
Regulatory assets, non current | 12.1 | 9.1
Spire Missouri | Other |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 10.2 | 7.5
Regulatory assets, non current | 42.7 | 27.2
Spire Missouri | Future Income Taxes Due from Customers |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, non current | 114.6 | 102.9
Spire Missouri | Accrued Cost of Removal |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, non current | 7.1 | 0.0
Spire Missouri | Energy Efficiency |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, non current | 39.6 | 35.0
Spire Alabama |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 20.4 | 33.9
Regulatory assets, non current | 489.9 | 231.2
Total Regulatory Assets | 510.3 | 265.1
Regulatory liabilities, current | 3.9 | 3.4
Regulatory liabilities, non current | 18.5 | 23.0
Total Regulatory Liabilities | 22.4 | 26.4
Spire Alabama | Pension and Other Postretirement Benefits |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 7.7 | 7.3
Regulatory assets, non current | 98.2 | 77.2
Spire Alabama | Unamortized Purchased Gas Adjustments |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 5.5 | 17.7
Regulatory assets, non current | 0.0 | 0.0
Spire Alabama | Other |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, current | 7.2 | 8.9
Regulatory assets, non current | 0.9 | 0.9
Spire Alabama | Future Income Taxes Due from Customers |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, non current | 2.2 | 2.2
Spire Alabama | Accrued Cost of Removal |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, non current | 388.6 | 150.9
Spire Alabama | Energy Efficiency |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory assets, non current | 0.0 | 0.0
Pension and Other Postretirement Benefits |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, current | 5.8 | 5.8
Regulatory liabilities, non current | 157.6 | 142.3
Pension and Other Postretirement Benefits | Spire Missouri |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, current | 3.6 | 3.6
Regulatory liabilities, non current | 140.4 | 119.1
Pension and Other Postretirement Benefits | Spire Alabama |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, current | 2.2 | 2.2
Regulatory liabilities, non current | 14.8 | 19.1
Unamortized Purchased Gas Adjustments |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, current | 73.1 | 26.2
Regulatory liabilities, non current | 4.4 | 0.0
Unamortized Purchased Gas Adjustments | Spire Missouri |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, current | 72.3 | 25.4
Regulatory liabilities, non current | 4.4 | 0.0
Unamortized Purchased Gas Adjustments | Spire Alabama |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, current | 0.0 | 0.0
Regulatory liabilities, non current | 0.0 | 0.0
Other |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, current | 34.1 | 28.8
Regulatory liabilities, non current | 14.3 | 35.3
Other | Spire Missouri |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, current | 27.3 | 23.3
Regulatory liabilities, non current | 8.6 | 29.2
Other | Spire Alabama |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, current | 1.7 | 1.2
Regulatory liabilities, non current | 3.7 | 3.9
Deferred Taxes Due to Customers |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, non current | 138.8 | 179.8
Deferred Taxes Due to Customers | Spire Missouri |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, non current | 121.4 | 162.5
Deferred Taxes Due to Customers | Spire Alabama |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, non current | 0.0 | 0.0
Accrued Cost of Removal |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, non current | 28.6 | 41.6
Accrued Cost of Removal | Spire Missouri |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, non current | 0.0 | 15.7
Accrued Cost of Removal | Spire Alabama |  |
Regulatory Asset And Liabilities [Line Items] |  |
Regulatory liabilities, non current | $ 0.0 | $ 0.0

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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