Financing - Schedule of Short-term Borrowings (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2025	Sep. 30, 2025	Dec. 31, 2024
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 1,399.2
Lowest borrowings outstanding	385.0
Weighted average borrowings	$ 1,036.3
Weighted average interest rate	4.10%
Borrowings outstanding	$ 412.0	$ 1,317.0	$ 1,158.0
Weighted average interest rate	3.90%	4.40%	4.40%
Spire Note [Member] \| Spire Missouri [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 594.7
Lowest borrowings outstanding	416.0
Weighted average borrowings	$ 485.8
Weighted average interest rate	4.20%
Borrowings outstanding	$ 465.7	$ 566.3	$ 580.9
Weighted average interest rate	3.90%	4.40%	4.80%
Spire Note [Member] \| Spire Alabama Inc [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 174.6
Lowest borrowings outstanding	120.3
Weighted average borrowings	$ 145.0
Weighted average interest rate	4.20%
Borrowings outstanding	$ 164.6	$ 130.1	$ 67.0
Weighted average interest rate	3.90%	4.40%	4.80%
Parent Company [Member] \| Commercial Paper [Member]
Short-Term Debt [Line Items]
Highest borrowings outstanding	$ 1,399.2
Lowest borrowings outstanding	385.0
Weighted average borrowings	$ 1,036.3
Weighted average interest rate	4.10%
Borrowings outstanding	$ 412.0	$ 1,317.0	$ 1,158.0
Weighted average interest rate	3.90%	4.40%	4.40%

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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, average borrowings during the period.

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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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