Note 1 - Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	6 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Allowance at beginning of period	$ 40.6	$ 35.1	$ 31.9	$ 30.3
Provision for expected credit losses	1.2	1.9	13.5	8.6
Write-offs, net of recoveries	(7.1)	(4.4)	(10.7)	(6.3)
Allowance at end of period	34.7	32.6	34.7	32.6
Spire Missouri [Member]
Allowance at beginning of period	34.5	27.0	24.9	22.6
Provision for expected credit losses	0.3	1.8	11.4	8.6
Write-offs, net of recoveries	(6.1)	(3.7)	(7.6)	(6.1)
Allowance at end of period	28.7	25.1	28.7	25.1
Spire Alabama Inc [Member]
Allowance at beginning of period	5.3	7.1	6.3	6.6
Provision for expected credit losses	0.8	0.0	1.5	0.0
Write-offs, net of recoveries	(0.8)	(0.5)	(2.5)	0.0
Allowance at end of period	$ 5.3	$ 6.6	$ 5.3	$ 6.6