Financing - Summary of Capitalized Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 6.3	$ 4.0	$ 12.6	$ 7.8
Spire Missouri [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	1.2	1.1	2.4	2.3
Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Interest Costs Capitalized	$ 1.0	$ 0.4	$ 1.8	$ 0.8