Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Accounting Policies [Line Items]
Allowance at beginning of period	$ 31.4	$ 32.5
Provision for expected credit losses	5.1	4.1
Write-offs, net of recoveries	(1.8)	(1.2)
Allowance at end of period	34.7	35.4
Spire Missouri [Member]
Accounting Policies [Line Items]
Allowance at beginning of period	24.9	26.2
Provision for expected credit losses	4.1	3.6
Write-offs, net of recoveries	(1.1)	(0.9)
Allowance at end of period	27.9	28.9
Spire Alabama Inc [Member]
Accounting Policies [Line Items]
Allowance at beginning of period	5.7	5.7
Provision for expected credit losses	0.8	0.3
Write-offs, net of recoveries	(0.5)	(0.3)
Allowance at end of period	$ 6.0	$ 5.7