SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property, Plant, And Equipment (Details)	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Spire Missouri
Property, Plant and Equipment [Line Items]
Average annual depreciation and amortization expense percentage	3.00%	3.00%	3.00%
Spire Alabama
Property, Plant and Equipment [Line Items]
Composite depreciation rate	3.10%	3.10%	3.10%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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