SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Capital Expenditure Excluded from Statement of Cash Flow (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Schedule of Accrued Capital Expenditures Excluded from Statement of Cash Flow [Line Items]
Accrued capital expenditures	$ 33.8	$ 26.5	$ 41.0
Spire Missouri
Schedule of Accrued Capital Expenditures Excluded from Statement of Cash Flow [Line Items]
Accrued capital expenditures	21.9	16.5	28.9
Spire Alabama
Schedule of Accrued Capital Expenditures Excluded from Statement of Cash Flow [Line Items]
Accrued capital expenditures	$ 9.9	$ 7.9	$ 9.4