SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Inter-company Transactions (Details) - Affiliated Entity - Spire Missouri - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Spire Marketing \| Regulated Operation
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	$ 10.4	$ 11.8	$ 52.0	$ 53.3
Sales of natural gas to Spire Marketing	0.0	1.5	0.3	7.8
Spire NGL Inc. \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received from Spire NGL Inc.	$ 0.3	$ 0.3	$ 0.8	$ 0.8

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
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-Section 50
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=114873854&loc=SL114874292-224272

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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

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