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, 2023	Sep. 30, 2022	Sep. 30, 2021
Accrued capital expenditures	$ 104.3	$ 77.8	$ 59.5
Spire Missouri [Member]
Accrued capital expenditures	56.5	45.6	37.1
Spire Alabama Inc [Member]
Accrued capital expenditures	$ 4.6	$ 19.2	$ 13.6