SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gross Receipts Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	$ 34.3	$ 32.3	$ 53.7	$ 50.2
Laclede Gas
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	25.5	24.7	39.6	38.6
Alagasco
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	$ 7.7	$ 7.6	$ 11.9	$ 11.6