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Divestitures - Additional Information (Details) - USD ($) $ in Millions				1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2026	Apr. 21, 2026	Apr. 14, 2026	Apr. 30, 2026	Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025	Sep. 30, 2025
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill					$ 1,891.4	$ 1,171.6	$ 1,891.4	$ 1,171.6	$ 1,171.6
Goodwill impairment					3.9	$ 0.0	3.9	$ 0.0
Cash proceeds from sale					30.0
Pre-tax gain recognized					$ 28.9
Spire Mississippi [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill impairment							3.9
Discontinued Operations [Member] \| Spire Marketing [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash proceeds from sale of business	$ 212.0
Discontinued Operations [Member] \| Spire Marketing [Member] \| Subsequent Event [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash proceeds from sale of business	212.0
Discontinued Operations [Member] \| Spire Storage [Member] \| Subsequent Event [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total expected consideration from transaction	650.0		$ 650.0	$ 650.0
Cash proceeds from sale of business			600.0	600.0
Deferred payment expected to be received in fiscal 2027	$ 50.0		$ 50.0	$ 50.0
Discontinued Operations [Member] \| Spire Mississippi [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill impairment							$ 3.9
Asset Held for Sale (Continuing Operations) [Member] \| Spire Mississippi [Member] \| Subsequent Event [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash purchase price		$ 75.0
Reverse termination fee		7.5
Goodwill		18.1
Goodwill impairment		$ 3.9