Note 2 - Revenue - Taxes Associated with Natural Gas Utility Services (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Gross Receipts Tax Amounts	$ 60.2	$ 51.9	$ 90.6	$ 73.8
Spire Missouri [Member]
Gross Receipts Tax Amounts	45.6	39.0	67.7	53.9
Spire Alabama Inc [Member]
Gross Receipts Tax Amounts	$ 12.6	$ 11.3	$ 19.4	$ 17.0