Note 13 - Pension Plans and Other Postretirement Benefits - PBO, ABO, and Fair Value of Plan Assets for Plans in Excess of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2022	Sep. 30, 2021
Projected benefit obligation	$ 464.4	$ 689.6
Accumulated benefit obligation	457.1	673.3
Fair value of plan assets	332.7	498.9
Spire Missouri [Member]
Projected benefit obligation	320.6	479.0
Accumulated benefit obligation	315.0	465.4
Fair value of plan assets	235.9	364.0
Spire Alabama Inc [Member]
Projected benefit obligation	97.8	149.4
Accumulated benefit obligation	96.2	146.9
Fair value of plan assets	$ 57.3	$ 82.8

Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e2417-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI https://asc.fasb.org/extlink&oid=123450688&loc=d3e4179-114921

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e2417-114920

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant