Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025
Accounting Policies [Line Items]
Allowance at beginning of period	$ 28.7	$ 34.7	$ 28.8	$ 31.4
Acquisition	1.7		1.7
Provision for expected credit losses	10.0	8.9	16.0	14.0
Write-offs, net of recoveries	(6.7)	(6.8)	(12.8)	(8.6)
Allowance at end of period	33.7	36.8	33.7	36.8
Spire Missouri [Member]
Accounting Policies [Line Items]
Allowance at beginning of period	22.5	27.9	23.0	24.9
Provision for expected credit losses	7.9	6.6	12.1	10.7
Write-offs, net of recoveries	(5.1)	(4.9)	(9.8)	(6.0)
Allowance at end of period	25.3	29.6	25.3	29.6
Spire Alabama Inc [Member]
Accounting Policies [Line Items]
Allowance at beginning of period	5.1	6.0	5.7	4.9
Provision for expected credit losses	1.7	2.0	3.2	2.8
Write-offs, net of recoveries	(1.2)	(1.9)	(2.5)	(2.4)
Allowance at end of period	$ 5.6	$ 6.1	$ 5.6	$ 6.1