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Derivative Instruments and Hedging Activities - Summary of NYMEX and ICE Natural Gas Futures and Swap Positions (Details) - MMBTU MMBTU in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024
Natural Gas Futures Purchased [Member] | Spire Missouri [Member] |  |
Derivative [Line Items] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 32.3 | 34.6
Derivative, hedge term (Year) | 12 months | 12 months
Natural Gas Futures Purchased [Member] | Gas Marketing [Member] |  |
Derivative [Line Items] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 48.0 | 56.0
Derivative, hedge term (Year) | 48 months | 48 months
Natural Gas Futures Purchased [Member] | Gas Utility [Member] |  |
Derivative [Line Items] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 32.3 | 34.6
Derivative, hedge term (Year) | 12 months | 12 months
Natural Gas Options Purchased Net [Member] | Gas Marketing [Member] |  |
Derivative [Line Items] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 9.9 | 3.8
Derivative, hedge term (Year) | 15 months | 15 months
Natural Gas Basis Swaps Purchased [Member] | Gas Marketing [Member] |  |
Derivative [Line Items] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 24.7 | 32.4
Derivative, hedge term (Year) | 39 months | 39 months

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