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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Projected Benefit Obligation (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Sep. 30, 2016
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Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | $ 748.8 | $ 794.7
Accumulated benefit obligation | 701.4 | 724.5
Fair value of plan assets | 531.6 | 540.5
Spire Missouri |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | 539.6 | 560.0
Accumulated benefit obligation | 500.4 | 517.7
Fair value of plan assets | 385.9 | 395.7
Spire Alabama |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | 148.2 | 174.3
Accumulated benefit obligation | 142.8 | 149.8
Fair value of plan assets | $ 97.9 | $ 100.0

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- Definition Amount of accumulated benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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- Definition Fair value of plan assets for defined benefit pension plan with accumulated benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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- Definition Amount of benefit obligation for defined benefit plan with benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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