Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Tax Credit Carryforward [Line Items]
Federal income tax statutory rate	35.00%	21.00%
Unrecognized tax benefits affect on the Company's effective tax rate		$ 2,900,000	$ 2,500,000
Income tax penalties accrued		0	0
Income taxes accrued		$ 0	$ 0
Spire
Tax Credit Carryforward [Line Items]
Federal income tax statutory rate		21.00%	21.00%	24.50%
Federal and state loss carryforwards		$ 726,800,000
Contribution carryforwards		9,300,000
Valuation allowance		3,600,000
Tax credit carryforwards		2,900,000
Tax credit carryforwards, valuation allowance		$ 100,000
Tax credit carryforwards, expiration year		2020
Spire | State
Tax Credit Carryforward [Line Items]
Operating loss carryforwards, expiration year		2030
Spire | Federal and Other States
Tax Credit Carryforward [Line Items]
Operating loss carryforwards, expiration year		2035
Spire Missouri
Tax Credit Carryforward [Line Items]
Federal income tax statutory rate		21.00%	21.00%	24.50%
Excess accumulated deferred taxes		$ 8,400,000	$ 8,400,000
Federal and state loss carryforwards		207,800,000
Contribution carryforwards		5,400,000
Valuation allowance		3,000,000.0
Tax credit carryforwards		2,000,000.0
Tax credit carryforwards, valuation allowance		$ 100,000
Tax credit carryforwards, expiration year		2020
Unrecognized tax benefits affect on the Company's effective tax rate		$ 2,700,000	$ 2,200,000
Spire Missouri | State
Tax Credit Carryforward [Line Items]
Operating loss carryforwards, expiration year		2035
Spire Missouri | Federal
Tax Credit Carryforward [Line Items]
Operating loss carryforwards, expiration year		2035
Spire Alabama
Tax Credit Carryforward [Line Items]
Federal income tax statutory rate		21.00%	21.00%	24.50%
Federal and state loss carryforwards		$ 443,500,000
Unrecognized tax benefits affect on the Company's effective tax rate		$ 0	$ 0	$ 0
Spire Alabama | State
Tax Credit Carryforward [Line Items]
Operating loss carryforwards, expiration year		2030
Spire Alabama | Federal
Tax Credit Carryforward [Line Items]
Operating loss carryforwards, expiration year		2035