Note 5 - Financing - Capitalized Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Interest Paid Capitalized	$ 1.1	$ 1.1	$ 3.2	$ 3.1
Spire Missouri [Member]
Interest Paid Capitalized	0.2	0.0	0.3	0.1
Spire Alabama Inc [Member]
Interest Paid Capitalized	$ 0.9	$ 0.8	$ 2.5	$ 2.1

Amount of cash paid for interest capitalized, classified as investing activity.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3213-108585

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 2
-URI https://asc.fasb.org/extlink&oid=126999549&loc=d3e4297-108586

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration