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Note 10 - 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, 2023 | Sep. 30, 2022
Natural Gas Futures Purchased [Member] | Spire Missouri [Member] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 66.5 | 13.0
Derivative, hedge term (Year) | 24 months | 12 months
Natural Gas Futures Purchased [Member] | Gas Marketing [Member] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 61.3 | 76.3
Derivative, hedge term (Year) | 51 months | 54 months
Natural Gas Futures Purchased [Member] | Gas Utility [Member] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 66.5 | 13.0
Derivative, hedge term (Year) | 24 months | 12 months
Natural Gas Options Purchased Net [Member] | Gas Marketing [Member] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 4.9 | 3.7
Derivative, hedge term (Year) | 15 months | 12 months
Natural Gas Basis Swaps Purchased [Member] | Gas Marketing [Member] |  |
Notional Amount, Energy Measure (Millions of British Thermal Unit) | 44.1 | 61.7
Derivative, hedge term (Year) | 51 months | 39 months

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- Definition Nominal energy measure used to calculate payments on a derivative instrument. + References No definition available. + Details
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Data Type: | dtr-types:energyItemType
Balance Type: | na
Period Type: | duration

X
- 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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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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