Leases - Operating Lease Cost Cash Flow and Noncash Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Lessee Lease Description [Line Items]
Operating lease cost, including amounts capitalized	$ 7.2	$ 8.7
Cash flow and noncash information about operating leases:
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	7.2	8.5
Right-of-use assets obtained in exchange for lease liabilities		71.1
Spire Missouri Inc
Lessee Lease Description [Line Items]
Operating lease cost, including amounts capitalized	0.4	0.5
Cash flow and noncash information about operating leases:
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	0.4	0.5
Right-of-use assets obtained in exchange for lease liabilities		2.1
Spire Alabama
Lessee Lease Description [Line Items]
Operating lease cost, including amounts capitalized	2.1	3.5
Cash flow and noncash information about operating leases:
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	$ 2.1	3.3
Right-of-use assets obtained in exchange for lease liabilities		$ 10.0