Note 1 - Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Allowance at beginning of year	$ 31.9	$ 30.3	$ 24.9
Provision for expected credit losses	16.6	11.6	14.7
Write-offs, net of recoveries	(16.0)	(10.0)	(9.3)
Allowance at end of year	32.5	31.9	30.3
Spire Missouri [Member]
Allowance at beginning of year	24.9	22.6	18.1
Provision for expected credit losses	13.3	11.2	11.1
Write-offs, net of recoveries	(12.0)	(8.9)	(6.6)
Allowance at end of year	26.2	24.9	22.6
Spire Alabama Inc [Member]
Allowance at beginning of year	6.3	6.6	5.5
Provision for expected credit losses	2.6	0.3	3.1
Write-offs, net of recoveries	(3.2)	(0.6)	(2.0)
Allowance at end of year	$ 5.7	$ 6.3	$ 6.6