Note 1 - Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Allowance at beginning of period	$ 31.9	$ 30.3
Provision for expected credit losses	4.1	1.5
Write-offs, net of recoveries	(1.5)	(0.4)
Allowance at end of period	34.5	31.4
Spire Missouri [Member]
Allowance at beginning of period	24.9	22.6
Provision for expected credit losses	3.8	2.1
Write-offs, net of recoveries	(0.5)	(0.9)
Allowance at end of period	28.2	23.8
Spire Alabama Inc [Member]
Allowance at beginning of period	6.3	6.6
Provision for expected credit losses	0.2	(0.5)
Write-offs, net of recoveries	(1.0)	0.6
Allowance at end of period	$ 5.5	$ 6.7