SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Transactions with Affiliates (Details) - Affiliated Entity - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Laclede Pipeline Company [Member] \| Unregulated Operation \| Spire Missouri
Related Party Transaction [Line Items]
Revenue from related parties	$ 1.0	$ 1.0
Spire Missouri \| Regulated Operation \| Spire marketing
Related Party Transaction [Line Items]
Revenue from related parties	7.8	1.9	$ 4.0
Spire marketing \| Regulated Operation \| Spire Missouri
Related Party Transaction [Line Items]
Revenue from related parties	$ 74.4	$ 46.3	$ 74.1

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=6488393&loc=d3e606610-122999

Name:	us-gaap_RevenueFromRelatedParties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration