Note 6 - Financing - Short-term Borrowings (Details) - USD ($) $ in Millions	9 Months Ended
Jun. 30, 2024	Sep. 30, 2023	Jun. 30, 2023
Highest borrowings outstanding	$ 1,275.0
Lowest borrowings outstanding	741.0
Weighted average borrowings	$ 944.9
Weighted average interest rate	5.70%
Borrowings outstanding	$ 771.0	$ 955.5	$ 557.6
Weighted average interest rate	5.60%	5.60%	5.50%
Term Loan [Member] \| Spire Missouri [Member]
Highest borrowings outstanding	$ 200.0
Lowest borrowings outstanding	0.0
Weighted average borrowings	$ 84.9
Weighted average interest rate	6.40%
Borrowings outstanding	$ 0.0	$ 0.0	$ 0.0
Weighted average interest rate	0.00%	0.00%	0.00%
Spire Note [Member] \| Spire Alabama Inc [Member]
Highest borrowings outstanding	$ 132.5
Lowest borrowings outstanding	5.5
Weighted average borrowings	$ 68.8
Weighted average interest rate	5.70%
Borrowings outstanding	$ 23.0	$ 124.1	$ 108.1
Weighted average interest rate	5.60%	5.60%	5.50%
Spire Note [Member] \| Spire Missouri [Member]
Highest borrowings outstanding	$ 643.6
Lowest borrowings outstanding	228.5
Weighted average borrowings	$ 431.9
Weighted average interest rate	5.60%
Borrowings outstanding	$ 418.3	$ 540.6	$ 228.1
Weighted average interest rate	5.60%	5.60%	5.50%
Parent Company [Member] \| Commercial Paper [Member]
Highest borrowings outstanding	$ 1,135.0
Lowest borrowings outstanding	554.6
Weighted average borrowings	$ 860.0
Weighted average interest rate	5.70%
Borrowings outstanding	$ 771.0	$ 955.5	$ 557.6
Weighted average interest rate	5.60%	5.60%	5.50%

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.

+ References

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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
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481544/470-10-50-7

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
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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.

+ Details

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