SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Inter-company Transactions (Details) - Affiliated Entity - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Regulated Operation \| Laclede Gas \| Spire Marketing
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	$ 20.5	$ 13.2
Regulated Operation \| Laclede Gas \| Laclede Insurance Risk Services
Related Party Transaction [Line Items]
Insurance services received from Laclede Insurance Risk Services	1.1	0.2
Regulated Operation \| Spire Marketing \| Laclede Gas
Related Party Transaction [Line Items]
Sales of natural gas to Spire Marketing	3.6	0.7
Unregulated Operation \| Laclede Gas \| Laclede Pipeline Company
Related Party Transaction [Line Items]
Transportation services received from Laclede Pipeline Company	$ 0.3	$ 0.3

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

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+ 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
-Glossary Related Parties
-URI http://asc.fasb.org/extlink&oid=16382449

Reference 3: 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

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