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Income Taxes - Additional Information (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Tax Credit Carryforward [Line Items] |  |  |  |
Federal income tax statutory rate | 35.00% | 21.00% |  |
Unrecognized tax benefits affect on the Company's effective tax rate |  | $ 2,900,000 | $ 2,500,000 |
Income tax penalties accrued |  | 0 | 0 |
Income taxes accrued |  | $ 0 | $ 0 |
Spire |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 24.50%
Federal and state loss carryforwards |  | $ 726,800,000 |  |
Contribution carryforwards |  | 9,300,000 |  |
Valuation allowance |  | 3,600,000 |  |
Tax credit carryforwards |  | 2,900,000 |  |
Tax credit carryforwards, valuation allowance |  | $ 100,000 |  |
Tax credit carryforwards, expiration year |  | 2020 |  |
Spire | State |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2030 |  |
Spire | Federal and Other States |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2035 |  |
Spire Missouri |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 24.50%
Excess accumulated deferred taxes |  | $ 8,400,000 | $ 8,400,000 |
Federal and state loss carryforwards |  | 207,800,000 |  |
Contribution carryforwards |  | 5,400,000 |  |
Valuation allowance |  | 3,000,000.0 |  |
Tax credit carryforwards |  | 2,000,000.0 |  |
Tax credit carryforwards, valuation allowance |  | $ 100,000 |  |
Tax credit carryforwards, expiration year |  | 2020 |  |
Unrecognized tax benefits affect on the Company's effective tax rate |  | $ 2,700,000 | $ 2,200,000 |
Spire Missouri | State |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2035 |  |
Spire Missouri | Federal |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2035 |  |
Spire Alabama |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 24.50%
Federal and state loss carryforwards |  | $ 443,500,000 |  |
Unrecognized tax benefits affect on the Company's effective tax rate |  | $ 0 | $ 0 | $ 0
Spire Alabama | State |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2030 |  |
Spire Alabama | Federal |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2035 |  |

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