Note 1 - Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Allowance at beginning of period	$ 35.1	$ 34.4	$ 30.3	$ 24.9
Provision for expected credit losses	1.9	1.9	8.6	12.6
Write-offs, net of recoveries	(4.4)	(1.8)	(6.3)	(3.0)
Allowance at end of period	32.6	34.5	32.6	34.5
Spire Missouri [Member]
Allowance at beginning of period	27.0	26.6	22.6	18.1
Provision for expected credit losses	1.8	1.8	8.6	10.8
Write-offs, net of recoveries	(3.7)	(1.0)	(6.1)	(1.5)
Allowance at end of period	25.1	27.4	25.1	27.4
Spire Alabama Inc [Member]
Allowance at beginning of period	7.1	6.5	6.6	5.5
Provision for expected credit losses	0.0	0.1	0.0	1.4
Write-offs, net of recoveries	(0.5)	(0.6)	0.0	(0.9)
Allowance at end of period	$ 6.6	$ 6.0	$ 6.6	$ 6.0