Summary of Significant Accounting Policies - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2021	Mar. 31, 2021
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	$ 28.9	$ 24.9
Provision for expected credit losses	6.6	10.7
Write-offs, net of recoveries	(1.1)	(1.2)
Allowance at end of period	34.4	34.4
Spire Missouri
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	21.4	18.1
Provision for expected credit losses	5.7	9.0
Write-offs, net of recoveries	(0.5)	(0.5)
Allowance at end of period	26.6	26.6
Spire Alabama
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	6.2	5.5
Provision for expected credit losses	0.6	1.3
Write-offs, net of recoveries	(0.3)	(0.3)
Allowance at end of period	$ 6.5	$ 6.5