Revenue - Schedule of Gross Receipts Taxes Associated Revenue (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	$ 56.5	$ 55.2	$ 86.4	$ 82.0
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	42.1	39.9	63.8	59.4
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	$ 12.5	$ 13.4	$ 19.3	$ 19.1

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