Note 1 - Summary of Significant Accounting Policies - Accrued Capital Expenditures Excluded From Capital Expenditures in the Statements of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Sep. 30, 2021
Accrued capital expenditures	$ 52.6	$ 41.5	$ 59.5
Spire Missouri [Member]
Accrued capital expenditures	39.2	27.6	37.1
Spire Alabama Inc [Member]
Accrued capital expenditures	$ 7.7	$ 6.7	$ 13.6

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
-Paragraph 4
-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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