Segment Information - Schedule of Financial Data Related to Gross Receipt Taxes and Capital Expenditures (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Property, Plant and Equipment [Line Items]
Capital Expenditures	$ 202.8	$ 260.6
Spire Missouri [Member]
Property, Plant and Equipment [Line Items]
Gross Receipt Taxes	21.7	19.5
Capital Expenditures	148.0	178.3
Spire Alabama Inc [Member]
Property, Plant and Equipment [Line Items]
Gross Receipt Taxes	6.8	5.7
Capital Expenditures	$ 41.3	$ 34.5