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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Capital Expenditure Excluded from Statement of Cash Flow (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016
Schedule of Accrued Capital Expenditures Excluded from Statement of Cash Flow [Line Items] |  |  |
Accrued capital expenditures | $ 31.8 | $ 15.3 | $ 41.0
Spire Missouri |  |  |
Schedule of Accrued Capital Expenditures Excluded from Statement of Cash Flow [Line Items] |  |  |
Accrued capital expenditures | 15.2 | 6.8 | 28.9
Spire Alabama |  |  |
Schedule of Accrued Capital Expenditures Excluded from Statement of Cash Flow [Line Items] |  |  |
Accrued capital expenditures | $ 7.0 | $ 5.6 | $ 9.4

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- Definition Future cash outflow to pay for purchases of fixed assets that have occurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
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