Leases - Operating Lease Cost Cash Flow and Noncash Information (Details) $ in Millions	3 Months Ended
	Dec. 31, 2019 USD ($)
Lessee Lease Description [Line Items]
Operating lease cost, including amounts capitalized	$ 2.4
Cash flow and noncash information about operating leases:
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	2.3
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	0.2
Cash flow and noncash information about operating leases:
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	0.2
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	1.0
Cash flow and noncash information about operating leases:
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	1.0
Right-of-use assets obtained in exchange for lease liabilities	$ 10.0