Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Accounting Policies [Line Items]
Allowance at beginning of period	$ 34.7	$ 35.4	$ 31.4	$ 32.5
Provision for expected credit losses	8.9	7.9	14.0	12.0
Write-offs, net of recoveries	(6.8)	(4.2)	(8.6)	(5.4)
Allowance at end of period	36.8	39.1	36.8	39.1
Spire Missouri [Member]
Accounting Policies [Line Items]
Allowance at beginning of period	27.9	28.9	24.9	26.2
Provision for expected credit losses	6.6	6.2	10.7	9.8
Write-offs, net of recoveries	(4.9)	(3.1)	(6.0)	(4.0)
Allowance at end of period	29.6	32.0	29.6	32.0
Spire Alabama Inc [Member]
Accounting Policies [Line Items]
Allowance at beginning of period	6.0	5.7	5.7	5.7
Provision for expected credit losses	2.0	1.4	2.8	1.7
Write-offs, net of recoveries	(1.9)	(0.9)	(2.4)	(1.2)
Allowance at end of period	$ 6.1	$ 6.2	$ 6.1	$ 6.2