Summary of Significant Accounting Policies - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance at beginning of year	$ 22.4	$ 18.3	$ 20.5
Additions charged to expense	16.9	14.6	10.6
Net deductions	(16.3)	(10.5)	(12.8)
Allowance at end of year	23.0	22.4	18.3
Spire Missouri
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance at beginning of year	16.0	14.1	16.1
Additions charged to expense	12.3	11.9	10.1
Net deductions	(13.4)	(10.0)	(12.1)
Allowance at end of year	14.9	16.0	14.1
Spire Alabama
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance at beginning of year	3.9	2.6	3.3
Additions charged to expense	4.7	2.1	0.2
Net deductions	(2.3)	(0.8)	(0.9)
Allowance at end of year	$ 6.3	$ 3.9	$ 2.6