SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounts Receivable and Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance at beginning of year	$ 18.3	$ 20.5	$ 14.2
Additions charged to expense	14.6	10.6	6.5
Net deductions	(10.5)	(12.8)	(0.2)
Allowance at end of year	22.4	18.3	20.5
Spire Missouri
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance at beginning of year	14.1	16.1	10.0
Additions charged to expense	11.9	10.1	6.2
Net deductions	(10.0)	(12.1)	(0.1)
Allowance at end of year	16.0	14.1	16.1
Spire Alabama
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance at beginning of year	2.6	3.3	4.2
Additions charged to expense	2.1	0.2	0.6
Net deductions	(0.8)	(0.9)	(1.5)
Allowance at end of year	$ 3.9	$ 2.6	$ 3.3