Leases - Schedule of Operating Lease Cost, Cash Flow and Noncash Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Operating lease cost, including amounts capitalized	$ 7.1	$ 7.3	$ 7.3
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	7.3	7.4	7.3
Right-of-use assets obtained in exchange for lease liabilities	0.0	0.0	0.8
Spire Missouri [Member]
Operating lease cost, including amounts capitalized	0.5	0.5	0.5
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	0.5	0.5	0.5
Right-of-use assets obtained in exchange for lease liabilities	0.0	0.0	0.8
Spire Alabama Inc [Member]
Operating lease cost, including amounts capitalized	2.0	2.0	2.0
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	2.2	2.1	2.1
Right-of-use assets obtained in exchange for lease liabilities	$ 0.0	$ 0.0	$ 0.0