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