Note 2 - Revenue - Taxes Associated with Natural Gas Utility Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Gross Receipts Tax Amounts	$ 22.4	$ 26.0	$ 113.5	$ 116.6
Spire Missouri [Member]
Gross Receipts Tax Amounts	16.3	18.9	82.8	86.6
Spire Alabama Inc [Member]
Gross Receipts Tax Amounts	$ 5.3	$ 6.1	$ 26.3	$ 25.5