Segment Information - Schedule of Financial Data Related to Gross Receipt Taxes and Capital Expenditures (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Property, Plant and Equipment [Line Items]
Capital Expenditures	$ 922.4	$ 861.3	$ 662.5
Spire Missouri [Member]
Property, Plant and Equipment [Line Items]
Gross Receipt Taxes	82.9	93.1	96.7
Capital Expenditures	641.2	553.0	447.5
Spire Alabama Inc [Member]
Property, Plant and Equipment [Line Items]
Gross Receipt Taxes	27.9	30.1	29.9
Capital Expenditures	$ 144.2	$ 112.8	$ 117.6