Financing -Schedule of Short-term Borrowings (Details) - USD ($) $ in Millions	9 Months Ended
Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020
Spire Alabama \| Spire Note
Short-term Debt [Line Items]
Weighted average borrowings	$ 39.7
Highest borrowings outstanding	$ 152.2
Weighted average interest rate	0.20%
Borrowings outstanding		$ 121.3	$ 85.6
Weighted average interest rate		0.20%	0.50%
Spire Missouri
Short-term Debt [Line Items]
Weighted average borrowings	$ 90.7
Highest borrowings outstanding	$ 250.0
Weighted average interest rate	0.80%
Borrowings outstanding	$ 250.0
Weighted average interest rate	0.70%
Spire Missouri \| Spire Note
Short-term Debt [Line Items]
Weighted average borrowings	$ 333.1
Lowest borrowings outstanding	95.3
Highest borrowings outstanding	$ 441.9
Weighted average interest rate	0.20%
Borrowings outstanding	$ 217.5	$ 301.2	$ 218.3
Weighted average interest rate	0.20%	0.20%	0.50%
Spire
Short-term Debt [Line Items]
Weighted average borrowings	$ 633.1
Lowest borrowings outstanding	390.0
Highest borrowings outstanding	$ 850.5
Weighted average interest rate	0.40%
Borrowings outstanding	$ 461.0	$ 648.0	$ 477.6
Weighted average interest rate	0.50%	0.60%	0.90%
Spire \| Term Loan
Short-term Debt [Line Items]
Weighted average borrowings	$ 42.3
Highest borrowings outstanding	$ 150.0
Weighted average interest rate	1.10%
Borrowings outstanding		$ 150.0	$ 150.0
Weighted average interest rate		1.10%	1.20%
Spire \| Commercial Paper Notes
Short-term Debt [Line Items]
Weighted average borrowings	$ 500.1
Lowest borrowings outstanding	140.0
Highest borrowings outstanding	$ 775.0
Weighted average interest rate	0.30%
Borrowings outstanding	$ 211.0	$ 498.0	$ 327.6
Weighted average interest rate	0.20%	0.20%	0.50%

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

No definition available.

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- Definition

The average effective interest rate during the reporting period.

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Average amount borrowed under the credit facility during the period.

+ References

No definition available.

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

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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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