Note 17 - Leases - Year-end Balance Sheet and Weighted-average Information About Operating Lease (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Weighted-average remaining lease term (in years) (Year)	14 years 1 month 6 days	15 years
Weighted-average discount rate	4.20%	4.10%
Spire Missouri [Member]
Weighted-average remaining lease term (in years) (Year)	5 years 7 months 6 days	5 years 1 month 6 days
Weighted-average discount rate	3.30%	2.20%
Spire Alabama Inc [Member]
Weighted-average remaining lease term (in years) (Year)	13 years 3 months 18 days	14 years 3 months 18 days
Weighted-average discount rate	3.70%	3.70%
Deferred Charges and Other Assets, Other [Member]
Right-of-use assets	$ 70.6	$ 73.7
Deferred Charges and Other Assets, Other [Member] | Spire Missouri [Member]
Right-of-use assets	2.1	1.7
Deferred Charges and Other Assets, Other [Member] | Spire Alabama Inc [Member]
Right-of-use assets	19.2	20.2
Other Current Liabilities [Member]
Lease liabilities, current	6.8	6.5
Other Current Liabilities [Member] | Spire Missouri [Member]
Lease liabilities, current	0.5	0.4
Other Current Liabilities [Member] | Spire Alabama Inc [Member]
Lease liabilities, current	1.9	1.9
Deferred Credits and Other Liabilities, Other [Member]
Lease liabilities, noncurrent	70.2	73.7
Deferred Credits and Other Liabilities, Other [Member] | Spire Missouri [Member]
Lease liabilities, noncurrent	1.8	1.5
Deferred Credits and Other Liabilities, Other [Member] | Spire Alabama Inc [Member]
Lease liabilities, noncurrent	$ 23.5	$ 24.6