For an unclassified balance sheet, the amount of fees and other revenue, excluding investment income receivable, earned but not yet received, which were recognized in conformity with revenue recognition criteria based on estimates or specific contractual terms.

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Percentage of voting equity interests acquired at the acquisition date in the business combination.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions	9 Months Ended
	Jun. 30, 2018 USD ($) reporting_unit	Dec. 31, 2017	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)
Related Party Transaction [Line Items]
Number of operating segments \| reporting_unit	2
Spire Missouri
Related Party Transaction [Line Items]
Accrued unbilled revenues	$ 31.5		$ 30.1	$ 31.9
Spire Alabama
Related Party Transaction [Line Items]
Accrued unbilled revenues	$ 1.7		$ 1.9	$ 6.0
Natural gas storage facility
Related Party Transaction [Line Items]
Voting interest acquired		80.00%