Financing - Summary of Capitalized Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 3.8	$ 4.3	$ 16.4	$ 12.1
Spire Missouri [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	1.0	1.2	3.4	3.5
Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 1.2	$ 0.3	$ 3.0	$ 1.1

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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+ Details