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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions		6 Months Ended
	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) reporting_unit	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)
Related Party Transaction [Line Items]
Number of operating segments \| reporting_unit		2
Spire Missouri
Related Party Transaction [Line Items]
Accrued unbilled revenues		$ 85.2	$ 30.1	$ 66.7
Spire Alabama
Related Party Transaction [Line Items]
Accrued unbilled revenues		$ 5.3	$ 1.9	$ 13.5
Ryckman Creek Resources, LLC
Related Party Transaction [Line Items]
Voting interest acquired	80.00%
Purchase price of acquisition	$ 24.8
Cash paid for acquisition	16.0
Promissory note for acquisition	10.0
Promissory note, value	$ 8.8