Note 2 - Revenue - Taxes Associated with Natural Gas Utility Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Gross Receipts Tax Amounts	$ 26.0	$ 23.3	$ 116.6	$ 97.1
Spire Missouri [Member]
Gross Receipts Tax Amounts	18.9	16.7	86.6	70.6
Spire Alabama Inc [Member]
Gross Receipts Tax Amounts	$ 6.1	$ 5.5	$ 25.5	$ 22.5