Note 1 - Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Allowance at beginning of period	$ 35.4	$ 34.5	$ 32.5	$ 31.9
Provision for expected credit losses	7.9	8.2	12.0	12.3
Write-offs, net of recoveries	(4.2)	(2.1)	(5.4)	(3.6)
Allowance at end of period	39.1	40.6	39.1	40.6
Spire Missouri [Member]
Allowance at beginning of period	28.9	28.2	26.2	24.9
Provision for expected credit losses	6.2	7.3	9.8	11.1
Write-offs, net of recoveries	(3.1)	(1.0)	(4.0)	(1.5)
Allowance at end of period	32.0	34.5	32.0	34.5
Spire Alabama Inc [Member]
Allowance at beginning of period	5.7	5.5	5.7	6.3
Provision for expected credit losses	1.4	0.5	1.7	0.7
Write-offs, net of recoveries	(0.9)	(0.7)	(1.2)	(1.7)
Allowance at end of period	$ 6.2	$ 5.3	$ 6.2	$ 5.3