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INCOME TAXES - Unrecognized Tax Benefit Reconciliation (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Unrecognized tax benefits [Roll Forward] |  |  |
Unrecognized tax benefits, beginning of year | $ 10.0 | $ 7.1 | $ 4.6
Increases related to tax positions taken in current year | 2.4 | 3.4 | 2.9
Reductions due to lapse of applicable statute of limitations | (1.4) | (0.5) | (0.4)
Unrecognized tax benefits, end of year | 11.0 | 10.0 | 7.1
Spire Missouri |  |  |
Unrecognized tax benefits [Roll Forward] |  |  |
Unrecognized tax benefits, beginning of year | 9.7 | 6.9 | 4.2
Increases related to tax positions taken in current year | 2.4 | 3.3 | 2.9
Reductions due to lapse of applicable statute of limitations | (1.4) | (0.5) | (0.2)
Unrecognized tax benefits, end of year | $ 10.7 | $ 9.7 | $ 6.9

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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