Note 1 - Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Allowance at beginning of period	$ 32.5	$ 31.9
Provision for expected credit losses	4.1	4.1
Write-offs, net of recoveries	(1.2)	(1.5)
Allowance at end of period	35.4	34.5
Spire Missouri [Member]
Allowance at beginning of period	26.2	24.9
Provision for expected credit losses	3.6	3.8
Write-offs, net of recoveries	(0.9)	(0.5)
Allowance at end of period	28.9	28.2
Spire Alabama Inc [Member]
Allowance at beginning of period	5.7	6.3
Provision for expected credit losses	0.3	0.2
Write-offs, net of recoveries	(0.3)	(1.0)
Allowance at end of period	$ 5.7	$ 5.5