SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gross Receipts Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	$ 18.0	$ 14.8	$ 71.7	$ 65.0
Laclede Gas
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	12.4	11.1	52.0	49.7
Alagasco
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	$ 4.2	$ 3.7	$ 16.1	$ 15.3