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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Transactions with Affiliates (Details) - Affiliated Entity - USD ($) $ in Millions | 12 Months Ended
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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

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- Definition 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. + References 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 + Details
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