Financing - Schedule of Short-term Borrowings (Details) - USD ($) $ in Millions	9 Months Ended
Jun. 30, 2026	Sep. 30, 2025	Jun. 30, 2025
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 2,019.0
Lowest borrowings outstanding	0.0
Weighted average borrowings	$ 1,069.9
Weighted average interest rate	4.20%
Borrowings outstanding	$ 1,263.5	$ 1,317.0	$ 1,009.5
Weighted average interest rate	4.10%	4.40%	4.30%
Spire Missouri [Member] \| Spire Note [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 594.7
Lowest borrowings outstanding	299.9
Weighted average borrowings	$ 426.3
Weighted average interest rate	4.10%
Borrowings outstanding	$ 443.5	$ 566.3	$ 398.7
Weighted average interest rate	4.10%	4.40%	4.70%
Spire Alabama Inc [Member] \| Spire Note [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 174.6
Lowest borrowings outstanding	69.5
Weighted average borrowings	$ 138.9
Weighted average interest rate	4.10%
Borrowings outstanding	$ 168.2	$ 130.1	$ 56.4
Weighted average interest rate	4.10%	4.40%	4.70%
Parent Company [Member] \| Commercial Paper [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 1,399.2
Lowest borrowings outstanding	0.0
Weighted average borrowings	$ 850.4
Weighted average interest rate	4.10%
Borrowings outstanding	$ 1,263.5	$ 1,317.0	$ 1,009.5
Weighted average interest rate	4.10%	4.40%	4.30%
Parent Company [Member] \| DDTL Agreement [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 800.0
Lowest borrowings outstanding	0.0
Weighted average borrowings	$ 219.4
Weighted average interest rate	4.60%
Borrowings outstanding	$ 0.0	$ 0.0	$ 0.0
Weighted average interest rate	0.00%

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

No definition available.

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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.

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- Definition

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

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- Definition

Weighted average interest rate of short-term debt outstanding calculated over time.

+ References

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.

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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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