Note 2 - Revenue - Taxes Associated with Natural Gas Utility Services (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Gross Receipts Tax Amounts	$ 131.8	$ 109.8	$ 94.0
Spire Missouri [Member]
Gross Receipts Tax Amounts	96.7	79.6	64.3
Spire Alabama Inc [Member]
Gross Receipts Tax Amounts	$ 29.9	$ 25.5	$ 25.1