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Derivative Instruments and Hedging Activities - Additional Information (Details) MMBTU in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Aug. 31, 2018 USD ($) | Sep. 30, 2020 USD ($) MMBTU | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2020 USD ($) MMBTU | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($)
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity contracts (in MMBtu) | MMBTU |  | 278.3 |  |  |  | 278.3 |  |
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2021 (in MMBtu) | MMBTU |  | 233.1 |  |  |  | 233.1 |  |
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2022 (in MMBtu) | MMBTU |  | 35.7 |  |  |  | 35.7 |  |
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2023 (in MMBtu) | MMBTU |  | 5.8 |  |  |  | 5.8 |  |
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2024 (in MMBtu) | MMBTU |  | 1.3 |  |  |  | 1.3 |  |
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2025 (in MMBtu) | MMBTU |  | 1.6 |  |  |  | 1.6 |  |
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2026 (in MMBtu) | MMBTU |  | 0.8 |  |  |  | 0.8 |  |
Loss on termination and settlement of interest rate swap |  | $ 0.3 |  |  |  |  |  |
Cash margin receivables not offset with derivatives |  | 7.6 |  |  |  | $ 7.6 | $ 8.1 |
Spire Missouri |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Cash margin receivables not offset with derivatives |  |  |  |  |  |  | $ 7.2 | $ 7.8
Interest Rate Swap with Fixed Interest Rate of 2.7675% |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative, term of contract | 3 years |  |  |  |  |  |  |
Derivative fixed interest rate | 2.7675% |  |  |  |  |  |  |
Notional amount | $ 100.0 |  |  |  |  |  |  |
Loss on termination and settlement of interest rate swap |  |  |  | $ 2.5 |  |  |  |
Interest Rate Swap with Fixed Interest Rate of 3.250% |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative, term of contract |  |  |  |  | 3 years |  |  |
Derivative fixed interest rate |  |  |  |  | 3.25% |  |  |
Notional amount |  |  |  |  | $ 100.0 |  |  |
Cumulative mark-to-market loss in accumulated other comprehensive loss |  | $ (53.7) |  |  |  | (53.7) |  |
Interest Rate Swap with Fixed Interest Rate of 0.921% to 1.3105% |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional amount |  |  |  | $ 150.0 |  |  |  |
Mark-to-market loss |  |  |  |  |  | $ (0.5) |  |
Interest Rate Swap with Fixed Interest Rate of 0.761% |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative, term of contract |  |  | 10 years |  |  |  |  |
Derivative fixed interest rate |  |  | 0.761% |  |  |  |  |
Notional amount |  |  | $ 40.0 |  |  |  |  |
Minimum [Member] | Interest Rate Swap with Fixed Interest Rate of 0.921% to 1.3105% |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative fixed interest rate |  |  |  | 0.921% |  |  |  |
Maximum | Interest Rate Swap with Fixed Interest Rate of 0.921% to 1.3105% |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative fixed interest rate |  |  |  | 1.3105% |  |  |  |

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- Definition Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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