Financing - Summary of Capitalized Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 3.9	$ 3.8	$ 10.6	$ 16.4
Spire Missouri [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	1.6	1.0	4.4	3.4
Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 2.0	$ 1.2	$ 5.7	$ 3.0

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