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Summary of Significant Accounting Policies - Transactions With Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Transaction amount | $ 32.0 | $ 32.5 | $ 32.0
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Alabama Inc [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Transaction amount | 0.0 | 0.0 | 0.0
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Missouri [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Transaction amount | 0.0 | 0.7 | 0.7
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Alabama Inc [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Transaction amount | 0.0 | 0.0 | 0.0
Unregulated Operation [Member] | Spire MoGas Pipeline [Member] | Spire Missouri [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Transaction amount | 7.0 | 4.8 | 0.0
Unregulated Operation [Member] | Spire MoGas Pipeline [Member] | Spire Alabama Inc [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Transaction amount | 0.0 | 0.0 | 0.0
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Purchases of natural gas from Spire Marketing | 23.8 | 13.9 | 57.4
Transaction amount | 0.1 | 1.7 | 0.4
Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Purchases of natural gas from Spire Marketing | 13.9 | 6.7 | 4.7
Transaction amount | $ 0.0 | $ 0.0 | $ 0.0

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