Note Payables and Credit Agreements - Schedule of Spire Missouri and Alabama Short-term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Spire
Short-term Debt [Line Items]
Weighted average borrowings	$ 576.2
Lowest borrowings outstanding	432.6
Highest borrowings outstanding	$ 856.6
Weighted average interest rate	1.70%
Borrowings outstanding	$ 648.0	$ 743.2
Weighted average interest rate	0.60%	2.30%
Revolving Credit Facility Borrowings \| Spire
Short-term Debt [Line Items]
Weighted average borrowings	$ 0.1
Highest borrowings outstanding	$ 23.1
Weighted average interest rate	1.90%
Commercial Paper Notes \| Spire
Short-term Debt [Line Items]
Weighted average borrowings	$ 476.1
Lowest borrowings outstanding	73.5
Highest borrowings outstanding	$ 856.6
Weighted average interest rate	1.70%
Borrowings outstanding	$ 498.0	$ 743.2
Weighted average interest rate	0.20%	2.30%
Term Loan \| Spire
Short-term Debt [Line Items]
Weighted average borrowings	$ 77.5
Highest borrowings outstanding	$ 150.0
Weighted average interest rate	1.60%
Borrowings outstanding	$ 150.0
Weighted average interest rate	1.10%
Spire Missouri \| Revolving Credit Facility Borrowings
Short-term Debt [Line Items]
Weighted average borrowings	$ 17.9
Highest borrowings outstanding	$ 185.1
Weighted average interest rate	1.90%
Spire Missouri \| Spire Note
Short-term Debt [Line Items]
Weighted average borrowings	$ 242.0
Lowest borrowings outstanding	16.0
Highest borrowings outstanding	$ 429.5
Weighted average interest rate	1.50%
Borrowings outstanding	$ 301.2	$ 386.4
Weighted average interest rate	0.20%	2.30%
Spire Alabama \| Revolving Credit Facility Borrowings
Short-term Debt [Line Items]
Weighted average borrowings	$ 4.6
Highest borrowings outstanding	$ 50.0
Weighted average interest rate	1.90%
Spire Alabama \| Spire Note
Short-term Debt [Line Items]
Weighted average borrowings	$ 89.0
Lowest borrowings outstanding	18.5
Highest borrowings outstanding	$ 161.3
Weighted average interest rate	1.40%
Borrowings outstanding	$ 121.3	$ 128.7
Weighted average interest rate	0.20%	2.30%

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

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- Definition

The average effective interest rate during the reporting period.

+ References

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-Publisher FASB
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Average amount borrowed under the credit facility during the period.

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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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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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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.

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No definition available.

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