Leases - Operating Lease Cost Cash Flow and Noncash Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2020
Lessee Lease Description [Line Items]
Operating lease cost, including amounts capitalized	$ 2.4	$ 4.8
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	4.5
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	0.1	0.3
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	0.3
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	1.1	2.1
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	1.9
Right-of-use assets obtained in exchange for lease liabilities		$ 10.0