Divestitures - Schedule of Cash Flows From Discontinued Operations (Details) - Discontinued Operations, Held-for-Sale [Member] - USD ($) $ in Millions	6 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Spire Marketing [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided by operating activities	$ 51.2	$ 30.8
Net cash used in investing activities	(51.3)	(30.8)
Spire Storage [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided by operating activities	38.4	23.4
Net cash used in investing activities	$ (20.1)	$ (78.3)

Amount of cash inflow (outflow) from investing activity attributable to discontinued operation. Investing activity includes, but is not limited to, making and collecting loan, acquiring and disposing of debt and equity instruments, property, plant, and equipment, and other productive assets.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 24
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-24

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 205
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5B
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483499/205-20-50-5B

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Amount of cash inflow (outflow) from operating activity attributable to discontinued operation. Operating activity includes, but is not limited to, transaction, adjustment, and change in value not defined as investing or financing activity.

+ References

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- 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 360
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3A
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482099/360-10-50-3A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 360
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3A
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482099/360-10-50-3A

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