SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gross Receipts and Sales Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	$ 98.2	$ 84.6	$ 75.5
Spire Missouri
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	68.9	60.7	57.4
Spire Alabama
Schedule of Gross Receipts Taxes [Line Items]
Gross Receipts taxes recorded in regulated gas distribution operating revenues	$ 25.4	$ 19.5	$ 17.9