Divestitures - Schedule of Assets and Liabilities Associated with Assets Held for Sale (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net allowance of account receivable	$ 33.7	$ 36.8	$ 33.7	$ 36.8	$ 28.7	$ 28.8	$ 34.7	$ 31.4
Goodwill impairment	3.9	0.0	3.9	0.0
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]
Non-utility property, accumulated depreciation	7.3	6.8	7.3	6.8		7.1
Discontinued Operations [Member] \| Spire Storage [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Non-utility property, accumulated depreciation	22.2	$ 9.7	22.2	$ 9.7		$ 15.7
Discontinued Operations [Member] \| Spire Mississippi [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net allowance of account receivable	$ 0.1		0.1
Goodwill impairment			$ 3.9

Amount of impairment loss from asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.