Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Accounting Policies [Line Items]
Allowance at beginning of period	$ 33.7	$ 36.8	$ 28.8	$ 31.4
Acquisition			1.7
Provision for expected credit losses	6.6	3.6	22.6	17.6
Write-offs, net of recoveries	(9.9)	(6.6)	(22.7)	(15.2)
Allowance at end of period	30.4	33.8	30.4	33.8
Spire Missouri [Member]
Accounting Policies [Line Items]
Allowance at beginning of period	25.3	29.6	23.0	24.9
Provision for expected credit losses	6.3	2.6	18.4	13.3
Write-offs, net of recoveries	(8.7)	(4.3)	(18.5)	(10.3)
Allowance at end of period	22.9	27.9	22.9	27.9
Spire Alabama Inc [Member]
Accounting Policies [Line Items]
Allowance at beginning of period	5.6	6.1	4.9	5.7
Provision for expected credit losses	0.3	0.7	3.5	3.5
Write-offs, net of recoveries	(1.0)	(1.7)	(3.5)	(4.1)
Allowance at end of period	$ 4.9	$ 5.1	$ 4.9	$ 5.1