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Note 1 - Summary of Significant Accounting Policies (Details Textual) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
	Jan. 19, 2024	Apr. 01, 2023	Dec. 31, 2023	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Number of Reportable Segments				3
Difference Between Replacement Cost of Gas Storage and Inventory Basis Amount				$ 27.3	$ 21.1
Goodwill				1,171.6	1,171.6
Payments to Acquire Businesses, Gross		$ 37.0
Restricted Cash				30.4	$ 20.2
Other Income [Member]
Gain on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring			$ 8.2
MoGas Pipeline [Member]
Payments to Acquire Businesses, Gross	$ 176.1
Gas Utility [Member]
Goodwill				$ 210.2
Spire Alabama Inc [Member]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service				3.00%	3.00%	3.00%
Financing Receivable, after Allowance for Credit Loss				$ 5.9	$ 6.7
Spire Alabama Inc [Member] \| Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, before Allowance for Credit Loss				$ 0.4	$ 0.3
Spire Missouri [Member]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service				3.00%	3.00%	3.00%
Goodwill				$ 210.2	$ 210.2