SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Capital Expenditure Excluded from Statement of Cash Flow (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Accrued Capital Expenditures Excluded from Statement of Cash Flow [Line Items]
Accrued capital expenditures	$ 31.8	$ 15.3	$ 41.0
Spire Missouri
Schedule of Accrued Capital Expenditures Excluded from Statement of Cash Flow [Line Items]
Accrued capital expenditures	15.2	6.8	28.9
Spire Alabama
Schedule of Accrued Capital Expenditures Excluded from Statement of Cash Flow [Line Items]
Accrued capital expenditures	$ 7.0	$ 5.6	$ 9.4