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, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Allowance at beginning of period	$ 34.5	$ 31.4	$ 31.9	$ 30.3
Provision for expected credit losses	8.2	5.2	12.3	6.7
Write-offs, net of recoveries	(2.1)	(1.5)	(3.6)	(1.9)
Allowance at end of period	40.6	35.1	40.6	35.1
Spire Missouri [Member]
Allowance at beginning of period	28.2	23.8	24.9	22.6
Provision for expected credit losses	7.3	4.7	11.1	6.8
Write-offs, net of recoveries	(1.0)	(1.5)	(1.5)	(2.4)
Allowance at end of period	34.5	27.0	34.5	27.0
Spire Alabama Inc [Member]
Allowance at beginning of period	5.5	6.7	6.3	6.6
Provision for expected credit losses	0.5	0.5	0.7	0.0
Write-offs, net of recoveries	(0.7)	(0.1)	(1.7)	0.5
Allowance at end of period	$ 5.3	$ 7.1	$ 5.3	$ 7.1