Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Tax Credit Carryforward [Line Items]
Unrecognized tax benefits affect on the Company's effective tax rate	$ 3,600,000	$ 2,900,000
Income tax penalties accrued	0	0
Income taxes accrued	0	0
Spire
Tax Credit Carryforward [Line Items]
Valuation allowance	$ 1,900,000
State | Spire
Tax Credit Carryforward [Line Items]
Operating loss carryforwards, expiration year	2030
Federal and Other States | Spire
Tax Credit Carryforward [Line Items]
Operating loss carryforwards, expiration year	2035
Spire Missouri
Tax Credit Carryforward [Line Items]
Excess accumulated deferred taxes	$ 8,400,000	8,400,000	$ 8,400,000
Valuation allowance	1,500,000
Unrecognized tax benefits affect on the Company's effective tax rate	3,300,000	2,700,000
Spire Alabama
Tax Credit Carryforward [Line Items]
Unrecognized tax benefits affect on the Company's effective tax rate	$ 0	$ 0	$ 0