Summary of Significant Accounting Policies - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	$ 30.3	$ 24.9
Provision for expected credit losses	1.5	4.1
Write-offs, net of recoveries	(0.4)	(0.1)
Allowance at end of period	31.4	28.9
Spire Missouri
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	22.6	18.1
Provision for expected credit losses	2.1	3.3
Write-offs, net of recoveries	(0.9)
Allowance at end of period	23.8	21.4
Spire Alabama
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	6.6	5.5
Provision for expected credit losses	(0.5)	0.7
Write-offs, net of recoveries	0.6
Allowance at end of period	$ 6.7	$ 6.2