SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gross Receipts Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Gross Receipts Taxes [Line Items]
Gross receipts sales taxes recorded as revenues	$ 20.4	$ 18.0	$ 87.1	$ 71.7
Spire Missouri
Schedule of Gross Receipts Taxes [Line Items]
Gross receipts sales taxes recorded as revenues	14.5	12.4	61.3	52.0
Spire Alabama
Schedule of Gross Receipts Taxes [Line Items]
Gross receipts sales taxes recorded as revenues	$ 5.2	$ 4.2	$ 22.4	$ 16.1