Leases - Schedule of Balance Sheet and Weighted Average Information About Operating Leases (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Right-of-use assets	$ 59.2	$ 63.1
Lease liabilities, current	6.1	6.6
Lease liabilities, noncurrent	$ 59.1	$ 62.7
Weighted-average remaining lease term (in years)	12 years 7 months 6 days	13 years 6 months
Weighted-average discount rate	4.20%	4.20%
Spire Missouri [Member]
Right-of-use assets	$ 1.6	$ 1.7
Lease liabilities, current	0.5	0.5
Lease liabilities, noncurrent	$ 1.2	$ 1.3
Weighted-average remaining lease term (in years)	4 years 9 months 18 days	5 years 1 month 6 days
Weighted-average discount rate	3.80%	3.40%
Spire Alabama Inc [Member]
Right-of-use assets	$ 17.0	$ 18.1
Lease liabilities, current	2.0	1.9
Lease liabilities, noncurrent	$ 20.9	$ 22.3
Weighted-average remaining lease term (in years)	11 years 3 months 18 days	12 years 3 months 18 days
Weighted-average discount rate	3.70%	3.70%