Summary of Significant Accounting Policies - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of year	$ 24.9	$ 23.0	$ 22.4
Provision for expected credit losses	14.7	14.0	16.9
Write-offs, net of recoveries	(9.3)	(12.1)	(16.3)
Allowance at end of year	30.3	24.9	23.0
Spire Missouri
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of year	18.1	14.9	16.0
Provision for expected credit losses	11.1	12.7	12.3
Write-offs, net of recoveries	(6.6)	(9.5)	(13.4)
Allowance at end of year	22.6	18.1	14.9
Spire Alabama
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of year	5.5	6.3	3.9
Provision for expected credit losses	3.1	0.9	4.7
Write-offs, net of recoveries	(2.0)	(1.7)	(2.3)
Allowance at end of year	$ 6.6	$ 5.5	$ 6.3