Note 1 - Summary of Significant Accounting Policies - Accrued Capital Expenditures Excluded From Capital Expenditures in the Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Accrued capital expenditures	$ 77.8	$ 59.5	$ 67.6
Spire Missouri [Member]
Accrued capital expenditures	45.6	37.1	34.3
Spire Alabama Inc [Member]
Accrued capital expenditures	$ 19.2	$ 13.6	$ 17.0

Future cash outflow to pay for purchases of fixed assets that have occurred.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI https://asc.fasb.org/extlink&oid=126999549&loc=d3e4332-108586

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
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-URI https://asc.fasb.org/extlink&oid=126999549&loc=d3e4313-108586

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI https://asc.fasb.org/extlink&oid=126999549&loc=d3e4304-108586

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