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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Targeted and Actual Plan Assets by Category (Details) | Sep. 30, 2017 | Sep. 30, 2016
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Spire Alabama | Pension Plans |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 100.00% | 100.00%
Actual allocation of plan assets | 100.00% | 100.00%
Spire Alabama | Pension Plans | Equity markets |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 60.00% | 60.00%
Actual allocation of plan assets | 58.50% | 59.20%
Spire Alabama | Pension Plans | Commodity Contract [Member] |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 29.00% | 29.00%
Actual allocation of plan assets | 28.70% | 28.80%
Spire Alabama | Pension Plans | Real Estate [Member] |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 11.00% | 11.00%
Actual allocation of plan assets | 12.80% | 12.00%
Spire Alabama | Postretirement Plans |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 100.00% |
Actual allocation of plan assets | 100.00% | 100.00%
Spire Alabama | Postretirement Plans | Equity markets |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 60.00% |
Actual allocation of plan assets | 60.10% | 60.50%
Spire Alabama | Postretirement Plans | Debt securities |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 40.00% |
Actual allocation of plan assets | 39.90% | 39.50%
Spire Missouri | Pension Plans |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 100.00% | 100.00%
Actual allocation of plan assets | 100.00% | 100.00%
Spire Missouri | Pension Plans | Equity markets |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 56.40% | 56.20%
Actual allocation of plan assets | 56.80% | 56.90%
Spire Missouri | Pension Plans | Debt securities |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 43.60% | 43.80%
Actual allocation of plan assets | 42.00% | 43.10%
Spire Missouri | Pension Plans | Other |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 0.00% | 0.00%
Actual allocation of plan assets | 1.20% | 0.00%
Spire Missouri | Postretirement Plans |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 100.00% |
Actual allocation of plan assets | 100.00% | 100.00%
Spire Missouri | Postretirement Plans | Equity markets |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 60.00% |
Actual allocation of plan assets | 59.00% | 59.10%
Spire Missouri | Postretirement Plans | Debt securities |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 40.00% |
Actual allocation of plan assets | 39.40% | 39.40%
Spire Missouri | Postretirement Plans | Other |  |
Targeted and actual plan assets by category [Abstract] |  |
Targeted allocation of plan assets | 0.00% |
Actual allocation of plan assets | 1.60% | 1.50%

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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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