Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2023 USD ($)	Sep. 30, 2025 USD ($) Segment	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)
Accounting Policies [Line Items]
Number of reportable segments | Segment		3
Difference between replacement cost of gas storage and inventory basis amount		$ 21.3	$ 27.3
Goodwill		1,171.6	1,171.6
Restricted Cash		35.5	30.4
Utility Plant [Member]
Accounting Policies [Line Items]
Utility plant, construction work-in-progress		268.3	143.2
Non-utility Property [Member]
Accounting Policies [Line Items]
Depreciation		$ 32.7	$ 25.8	$ 20.6
Spire Alabama Inc [Member]
Accounting Policies [Line Items]
Number of reportable segments | Segment		1
Public utilities, property, plant and equipment, disclosure of composite depreciation rate for plants in service		3.00%	3.00%	3.00%
Financing Receivable, after Allowance for Credit Loss		$ 5.4	$ 5.9
Gas Utility [Member]
Accounting Policies [Line Items]
Goodwill		210.2
Gas Marketing [Member]
Accounting Policies [Line Items]
Goodwill		0.0
Midstream [Member]
Accounting Policies [Line Items]
Goodwill		0.0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Spire Alabama Inc [Member]
Accounting Policies [Line Items]
Financing Receivable, before Allowance for Credit Loss		$ 0.4	$ 0.4
Other Income [Member]
Accounting Policies [Line Items]
Gain on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring	$ 8.2