Financing - Summary of Capitalized Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 6.3	$ 3.8
Spire Missouri [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	1.2	1.2
Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 0.8	$ 0.4