Note 2 - Revenue - Taxes Associated With Natural Gas Utility Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Gross Receipts Tax Amounts	$ 23.3	$ 17.9	$ 97.1	$ 81.8
Spire Missouri [Member]
Gross Receipts Tax Amounts	16.7	12.3	70.6	56.1
Spire Alabama Inc [Member]
Gross Receipts Tax Amounts	$ 5.5	$ 4.7	$ 22.5	$ 21.8