Note 1 - Summary of Significant Accounting Policies - Accrued Capital Expenditures Excluded from Capital Expenditures in the Statements of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2022
Accrued capital expenditures	$ 66.3	$ 38.8	$ 77.8
Spire Missouri [Member]
Accrued capital expenditures	35.4	28.7	45.6
Spire Alabama Inc [Member]
Accrued capital expenditures	$ 5.2	$ 5.0	$ 19.2