Summary of Significant Accounting Policies - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2021	Jun. 30, 2021
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	$ 34.4	$ 24.9
Provision for expected credit losses	1.9	12.6
Write-offs, net of recoveries	(1.8)	(3.0)
Allowance at end of period	34.5	34.5
Spire Missouri
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	26.6	18.1
Provision for expected credit losses	1.8	10.8
Write-offs, net of recoveries	(1.0)	(1.5)
Allowance at end of period	27.4	27.4
Spire Alabama
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	6.5	5.5
Provision for expected credit losses	0.1	1.4
Write-offs, net of recoveries	(0.6)	(0.9)
Allowance at end of period	$ 6.0	$ 6.0