Note 2 - Revenue - Taxes Associated with Natural Gas Utility Services (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Gross Receipts Tax Amounts	$ 31.1	$ 30.4
Spire Missouri [Member]
Gross Receipts Tax Amounts	22.5	22.1
Spire Alabama Inc [Member]
Gross Receipts Tax Amounts	$ 6.9	$ 6.8