Revenue - Schedule of Gross Receipts Taxes Associated Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	$ 19.6	$ 22.4	$ 101.6	$ 113.5
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	13.9	16.3	73.3	82.8
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	$ 5.0	$ 5.3	$ 24.1	$ 26.3

- Definition

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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