Note 1 - Summary of Significant Accounting Policies (Details Textual) - USD ($) $ in Millions		12 Months Ended
Apr. 01, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Number of Reportable Segments		3
Difference Between Replacement Cost of Gas Storage and Inventory Basis Amount		$ 21.1	$ 37.3
Goodwill		1,171.6	1,171.6
Payments to Acquire Businesses, Gross	$ 37.0	37.0	(0.0)	$ (0.0)
Restricted Cash		20.2	$ 14.0
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		$ 6.7	$ 7.1
Spire Alabama Inc [Member] \| Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, before Allowance for Credit Loss		$ 0.3	$ 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

The amount by which the replacement cost exceeds (is less than) the cost basis value of stored gas inventories..

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amortized cost, before allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement and net investment in lease.

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Amortized cost, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement and net investment in lease.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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Composite depreciation rate for public utility plants in service.

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Amount of cash restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits.

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