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, 2023	Jun. 30, 2022	Sep. 30, 2022
Accrued capital expenditures	$ 62.2	$ 52.6	$ 77.8
Spire Missouri [Member]
Accrued capital expenditures	33.4	39.2	45.6
Spire Alabama Inc [Member]
Accrued capital expenditures	$ 4.5	$ 7.7	$ 19.2