Note 6 - Financing - Short-term Borrowings (Details) - USD ($) $ in Millions	9 Months Ended
Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2022
Highest borrowings outstanding	$ 1,482.0
Lowest borrowings outstanding	489.0
Weighted average borrowings	$ 868.1
Weighted average interest rate	4.80%
Borrowings outstanding	$ 557.6	$ 1,037.5	$ 709.2
Weighted average interest rate	5.50%	3.30%	2.00%
Spire Missouri [Member] \| Term Loan [Member]
Highest borrowings outstanding	$ 250.0
Lowest borrowings outstanding	0.0
Weighted average borrowings	$ 89.4
Weighted average interest rate	5.50%
Borrowings outstanding	$ 0.0	$ 0.0	$ 0.0
Weighted average interest rate	0.00%
Spire Missouri [Member] \| Spire Note [Member]
Highest borrowings outstanding	$ 656.9
Lowest borrowings outstanding	3.8
Weighted average borrowings	$ 308.9
Weighted average interest rate	4.60%
Borrowings outstanding	$ 228.1	$ 445.3	$ 285.4
Weighted average interest rate	5.50%	3.30%	2.00%
Spire Alabama Inc [Member] \| Spire Note [Member]
Highest borrowings outstanding	$ 274.0
Lowest borrowings outstanding	76.7
Weighted average borrowings	$ 123.3
Weighted average interest rate	4.80%
Borrowings outstanding	$ 108.1	$ 260.9	$ 196.3
Weighted average interest rate	5.50%	3.30%	2.00%
Commercial Paper [Member] \| Parent Company [Member]
Highest borrowings outstanding	$ 1,282.0
Lowest borrowings outstanding	354.4
Weighted average borrowings	$ 778.7
Weighted average interest rate	4.70%
Borrowings outstanding	$ 557.6	$ 1,037.5	$ 709.2
Weighted average interest rate	5.50%	3.30%	2.00%

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 852
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Period Type:	instant

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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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
-Topic 946
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-Name Accounting Standards Codification
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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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