Financing - Summary of Capitalized Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 3.4	$ 6.3	$ 6.7	$ 12.6
Spire Missouri [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	1.4	1.2	2.8	2.4
Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 2.0	$ 1.0	$ 3.7	$ 1.8

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