SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Spire Missouri
Related Party Transaction [Line Items]
Accrued unbilled revenues	$ 29.7	$ 30.1
Spire Alabama
Related Party Transaction [Line Items]
Accrued unbilled revenues	$ 0.0	$ 1.9

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_AccruedFeesAndOtherRevenueReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant