Note 2 - Revenue - Taxes Associated with Natural Gas Utility Services (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Gross Receipts Tax Amounts	$ 60.0	$ 60.2	$ 91.1	$ 90.6
Spire Missouri [Member]
Gross Receipts Tax Amounts	44.0	45.6	66.5	67.7
Spire Alabama Inc [Member]
Gross Receipts Tax Amounts	$ 14.1	$ 12.6	$ 21.0	$ 19.4