Summary of Significant Accounting Policies - Schedule of Allowance for Credit Losses (Details) $ in Millions	3 Months Ended
	Dec. 31, 2020 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	$ 24.9
Provision for expected credit losses	4.1
Write-offs, net of recoveries	(0.1)
Allowance at end of period	28.9
Spire Missouri
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	18.1
Provision for expected credit losses	3.3
Allowance at end of period	21.4
Spire Alabama
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	5.5
Provision for expected credit losses	0.7
Allowance at end of period	$ 6.2