Leases - Operating Lease Cost Cash Flow and Noncash Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2020	Jun. 30, 2020
Lessee Lease Description [Line Items]
Operating lease cost, including amounts capitalized	$ 2.1	$ 6.9
Cash flow and noncash information about operating leases:
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	2.2	6.7
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	0.1	0.4
Cash flow and noncash information about operating leases:
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	0.1	0.4
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	0.9	3.0
Cash flow and noncash information about operating leases:
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities	$ 0.9	2.8
Right-of-use assets obtained in exchange for lease liabilities		$ 10.0