Notes Payable and Credit Agreements - Short-term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 1,348.0
Lowest borrowings outstanding	896.0
Weighted average borrowings	$ 1,085.7
Weighted average interest rate	4.40%
Borrowings outstanding	$ 1,317.0	$ 947.0
Weighted average interest rate	4.50%	5.20%
Spire Note [Member] \| Spire Missouri [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 615.0
Lowest borrowings outstanding	299.5
Weighted average borrowings	$ 482.6
Weighted average interest rate	4.40%
Borrowings outstanding	$ 566.3	$ 495.3
Weighted average interest rate	4.70%	5.20%
Spire Note [Member] \| Spire Alabama Inc [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 130.6
Lowest borrowings outstanding	1.2
Weighted average borrowings	$ 50.8
Weighted average interest rate	4.40%
Borrowings outstanding	$ 130.1	$ 48.4
Weighted average interest rate	4.70%	5.20%
Parent Company [Member] \| Commercial Paper [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 1,348.0
Lowest borrowings outstanding	896.0
Weighted average borrowings	$ 1,085.7
Weighted average interest rate	4.40%
Borrowings outstanding	$ 1,317.0	$ 947.0
Weighted average interest rate	4.50%	5.20%

Minimum amount borrowed under the credit facility at any time during the period.

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- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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- 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

No definition available.

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- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
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Weighted average interest rate of short-term debt outstanding calculated over time.

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No definition available.

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- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, average borrowings during the period.

+ References

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- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, the maximum amount borrowed at any time during the period.

+ References

No definition available.

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