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Regulatory Matters - Spire Missouri - Additional Information (Details) $ / shares in Units, $ in Millions |  |  |  |  |  | 1 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 12, 2020 USD ($) | Nov. 16, 2019 USD ($) | May 25, 2019 USD ($) | Apr. 25, 2018 USD ($) | Mar. 07, 2018 USD ($) $ / shares | Aug. 31, 2020 USD ($) | Mar. 31, 2020 USD ($) | Sep. 30, 2020 USD ($) operating-segment | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Apr. 18, 2018
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Customer Share |  |  |  |  |  |  |  | 75.00% |  |  |
Regulatory costs |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 30.6 |
Provision for ISRS rulings |  |  |  |  |  |  |  | 0.0 | 12.2 | 0.0 |
Regulatory assets |  |  |  |  |  |  |  | $ 69.5 | 78.6 |  |
Minimum |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Customer Share |  |  |  |  |  |  |  |  |  |  | 70.00%
Maximum |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Customer Share |  |  |  |  |  |  |  |  |  |  | 85.00%
MoPSC | Infrastructure System Replacement Surcharge |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease), costs from buildings sold in 2014 excluded from rate base |  |  |  | $ (1.8) |  |  |  |  |  |  |
Rate case expenses |  |  |  | (0.9) |  |  |  |  |  |  |
Regulatory costs |  |  |  |  |  |  |  |  |  | $ 38.4 |
After-tax rate reduction to net income |  |  |  |  | $ 23.6 |  |  |  |  |  |
After-tax rate reduction to earnings per share | $ / shares |  |  |  |  | $ 0.49 |  |  |  |  |  |
MoPSC | Infrastructure System Replacement Surcharge | Pension Cost |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Regulatory assets recovery |  |  |  | $ (28.8) |  |  |  |  |  |  |
Regulatory costs |  |  |  |  | $ 30.6 |  |  |  |  |  |
Spire Missouri |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of PGA Discretionary Changes | operating-segment |  |  |  |  |  |  |  | 3 |  |  |
Months between PGA Discretionary Changes |  |  |  |  |  |  |  | 2 months |  |  |
Customer Share |  |  |  |  |  |  |  | 25.00% |  |  |
Regulatory assets |  |  |  |  |  |  |  | $ 32.1 | 29.4 |  |
Spire Missouri | MoPSC |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cost savings retention percentage |  |  |  |  |  |  |  | 10.00% |  |  |
Maximum amount of cost savings allowable |  |  |  |  |  |  |  | $ 3.0 |  |  |
Spire Missouri | MoPSC | Infrastructure System Replacement Surcharge |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Approved rate authorized future annualized revenue |  |  |  |  |  |  |  | 40.3 |  |  |
Payment of customer refund |  |  |  |  |  | $ 15.0 |  |  |  |  |
Provision for ISRS rulings |  |  |  |  |  |  |  |  | $ 12.2 |  |
Additional loss contingency provision of customer refund |  |  |  |  |  |  | $ 4.8 |  |  |  |
Annual authorized revenue |  | $ 8.8 | $ 12.4 |  |  |  |  |  |  |  |
Spire Missouri | MoPSC | Infrastructure System Replacement Surcharge | Subsequent Event |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Approved rate additional annualized revenue | $ 7.0 |  |  |  |  |  |  |  |  |  |
Spire Missouri | MoPSC | Accounting Authority Order |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  |  | $ 3.8 |  |  |

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