Note 5 - Financing - Capitalized Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2022	Dec. 31, 2021
Interest Paid Capitalized	$ 1.6	$ 1.1
Spire Missouri [Member]
Interest Paid Capitalized	0.5	0.0
Spire Alabama Inc [Member]
Interest Paid Capitalized	$ 0.7	$ 0.8

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

Name:	us-gaap_InterestPaidCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration