PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Projected Benefit Obligation (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
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

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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

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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

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