SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gross Receipts Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	$ 23.1	$ 19.4
Spire Missouri
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	16.2	14.1
Spire Alabama
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	$ 5.6	$ 4.2