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Regulatory Matters - Additional Information (Details) - USD ($) $ in Millions |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 11, 2020 | Apr. 25, 2018 | Feb. 28, 2021 | Mar. 31, 2021 | Mar. 31, 2020 | Mar. 31, 2021 | Mar. 31, 2020 | Sep. 30, 2020 | Feb. 09, 2021
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Remaining recovery period for certain regulatory assets for which no return on investment during recovery period is provided (in years) |  |  |  |  |  | 20 years |  |  |
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided (in years) |  |  |  |  |  | 15 years |  |  |
Regulatory assets, current |  |  |  | $ 60.4 | $ 68.0 | $ 60.4 | $ 68.0 | $ 69.5 |
Natural gas |  |  | $ 195.8 | 619.1 | 297.1 | 800.3 | 559.0 |  |
Spire Missouri |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Regulatory assets, current |  |  |  | 37.5 | 29.4 | 37.5 | 29.4 | 32.1 |
Natural gas |  |  |  | 537.4 | $ 223.6 | 699.0 | 409.4 |  |
Incremental gross revenue |  |  | 100.0 |  |  |  |  |  |
Regulatory liability with a reduction in revenue |  |  | $ 25.0 |  |  |  |  |  |
Approved percentage of excess natural gas supply and capacity retain |  |  | 25.00% |  |  |  |  |  |
Approved percentage of excess natural gas supply and capacity to customers |  |  | 75.00% |  |  |  |  |  |
Change in net deferred gas costs and average inventory costs |  |  |  | 110.0 |  |  |  |  |
MoPSC |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Approved rate increase (decrease) amount | $ 64.2 |  |  |  |  |  |  | 2,780.0 |
Requested rate increase | $ 111.5 |  |  |  |  |  |  |  |
Common equity ratio | 54.25% |  |  |  |  |  |  |  |
Requested return rate on equity | 9.95% |  |  |  |  |  |  |  |
Infrastructure System Replacement Surcharge | MoPSC |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Accrued pension assets added to prepaid pension asset |  |  |  |  |  |  |  |  | $ 9.0
Requested rate increase | $ 47.3 |  |  |  |  |  |  |  |
Infrastructure System Replacement Surcharge | MoPSC | Spire Missouri |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Provision for regulatory liability |  |  |  |  |  |  | 4.2 |  |
Provision for interest |  |  |  |  |  |  | 0.6 |  |
After-tax impact on loss contingency provision reducing net income |  |  |  |  |  |  | $ 3.7 |  |
Accounting Authority Order | MoPSC |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Regulatory assets, current |  |  |  | $ 3.4 |  | $ 3.4 |  | $ 3.8 |
Pension Cost | Infrastructure System Replacement Surcharge | MoPSC |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Regulatory assets recovery |  | $ (28.8) |  |  |  |  |  |  |

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