Leases - Summary of Balance Sheet and Weighted-Average Information about Operating Leases (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Lessee Lease Description [Line Items]
Right-of-use assets	$ 60.4	$ 65.1
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	us-gaap:OtherAssets	us-gaap:OtherAssets
Lease liabilities, current	$ 6.5	$ 6.5
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other	Other
Lease liabilities, noncurrent	$ 53.7	$ 58.4
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other	Other
Weighted-average remaining lease term	15 years 3 months 18 days	15 years 10 months 24 days
Weighted-average discount rate	4.20%	4.20%
Spire Missouri Inc
Lessee Lease Description [Line Items]
Right-of-use assets	$ 1.4	$ 1.7
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	us-gaap:OtherAssets	us-gaap:OtherAssets
Lease liabilities, current	$ 0.3	$ 0.3
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other	Other
Lease liabilities, noncurrent	$ 1.0	$ 1.4
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other	Other
Weighted-average remaining lease term	4 years 6 months	5 years 3 months 18 days
Weighted-average discount rate	2.50%	2.50%
Spire Alabama
Lessee Lease Description [Line Items]
Right-of-use assets	$ 4.8	$ 6.7
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	us-gaap:OtherAssets	us-gaap:OtherAssets
Lease liabilities, current	$ 1.9	$ 1.9
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other	Other
Lease liabilities, noncurrent	$ 2.7	$ 4.7
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other	Other
Weighted-average remaining lease term	2 years 3 months 18 days	3 years 3 months 18 days
Weighted-average discount rate	2.20%	2.20%