Financing Arrangements and Long-term Debt - Schedule of Spire Missouri and Alabama Short-term Borrowings (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Spire Alabama \| Spire Note
Short-term Debt [Line Items]
Weighted average borrowings	$ 111.6
Lowest borrowings outstanding	1.5
Highest borrowings outstanding	$ 152.2
Weighted average interest rate	0.20%
Borrowings outstanding	$ 27.0	$ 121.3	$ 67.1
Weighted average interest rate	0.30%	0.20%	2.10%
Spire Missouri \| Spire Note
Short-term Debt [Line Items]
Weighted average borrowings	$ 341.3
Lowest borrowings outstanding	292.8
Highest borrowings outstanding	$ 396.7
Weighted average interest rate	0.20%
Borrowings outstanding	$ 393.7	$ 301.2	$ 288.1
Weighted average interest rate	0.30%	0.20%	2.10%
Spire
Short-term Debt [Line Items]
Weighted average borrowings	$ 734.7
Lowest borrowings outstanding	631.1
Highest borrowings outstanding	$ 850.5
Weighted average interest rate	0.40%
Borrowings outstanding	$ 696.1	$ 648.0	$ 518.9
Weighted average interest rate	0.30%	0.60%	2.10%
Spire \| Term Loan
Short-term Debt [Line Items]
Weighted average borrowings	$ 125.5
Highest borrowings outstanding	$ 150.0
Weighted average interest rate	1.10%
Borrowings outstanding		$ 150.0
Weighted average interest rate		1.10%
Spire \| Commercial Paper Notes
Short-term Debt [Line Items]
Weighted average borrowings	$ 609.2
Lowest borrowings outstanding	497.0
Highest borrowings outstanding	$ 701.1
Weighted average interest rate	0.20%
Borrowings outstanding	$ 696.1	$ 498.0	$ 518.9
Weighted average interest rate	0.30%	0.20%	2.10%

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The average effective interest rate during the reporting period.

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Average amount borrowed under the credit facility during the period.

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