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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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of liabilities incurred by the acquirer as part of consideration transferred in a 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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The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions		3 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) reporting_unit	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Related Party Transaction [Line Items]
Number of operating segments \| reporting_unit		2
Spire Missouri
Related Party Transaction [Line Items]
Accrued unbilled revenues	$ 116.2	$ 116.2	$ 30.1	$ 103.5
Spire Alabama
Related Party Transaction [Line Items]
Accrued unbilled revenues	$ 13.2	$ 13.2	$ 1.9	$ 22.0
Ryckman Creek Resources, LLC
Related Party Transaction [Line Items]
Voting interest acquired	80.00%	80.00%
Purchase price of acquisition	$ 26.0
Cash paid for acquisition	16.0
Promissory note for acquisition	$ 10.0