Leases - Additional Information (Details)	9 Months Ended
	Jun. 30, 2020
Lessee Lease Description [Line Items]
Operating lease, existence of option to extend	true
Operating lease, option to extend	The lease agreement covering Spire Marketing and Spire Storage office space in Houston extends through December 2028, with options to terminate three years earlier or to renew for an additional five years
Operating lease extend period	2028-12
Operating lease, renewal term	5 years
Operating lease, existence of option to terminate	true
Operating lease, existence of option to terminate period	with options to terminate three years
Spire Missouri
Lessee Lease Description [Line Items]
Operating lease, existence of option to extend	true
Operating lease, option to extend	The lease agreement covering the Company’s primary office space in St. Louis extends through February 2035, with an option to renew for an additional five years
Operating lease extend period	2035-02
Operating lease, renewal term	5 years
Spire Alabama
Lessee Lease Description [Line Items]
Operating lease, existence of option to extend	true
Operating lease, option to extend	Spire Alabama’s lease agreement for office space in Birmingham extends through January 2024.
Operating lease extend period	2024-01