Financing - Summary of Capitalized Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 3.3	$ 6.3
Spire Missouri [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	1.4	1.2
Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 1.7	$ 0.8