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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
	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

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 20
-Name Accounting Standards Codification
-Section 25
-Paragraph 6A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480682/815-20-25-6A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

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- Definition

Nominal energy measure used to calculate payments on a derivative instrument.

+ References

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- 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

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