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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Changes in Goodwill (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Goodwill [Line Items] |  |  |
Accruals for capital expenditures | $ 41.0 | $ 30.4 | $ 13.4
Goodwill [Roll Forward] |  |  |
Beginning balance | 1,164.9 | 946.0 | 937.8
Goodwill | 6.7 |  | 8.2
Acquisition |  | 218.9 |
Ending balance | 1,171.6 | 1,164.9 | 946.0
Operating Segments | Gas Utility: |  |  |
Goodwill [Roll Forward] |  |  |
Beginning balance | 210.2 | 210.2 | 210.2
Goodwill | 0.0 |  | 0.0
Acquisition |  | 0.0 |
Ending balance | 210.2 | 210.2 | 210.2
Operating Segments | Gas Marketing: |  |  |
Goodwill [Roll Forward] |  |  |
Beginning balance | 0.0 | 0.0 | 0.0
Goodwill | 0.0 |  | 0.0
Acquisition |  | 0.0 |
Ending balance | 0.0 | 0.0 | 0.0
Other |  |  |
Goodwill [Roll Forward] |  |  |
Beginning balance | 954.7 | 735.8 | 727.6
Goodwill | 6.7 |  | 8.2
Acquisition |  | 218.9 |
Ending balance | 961.4 | 954.7 | 735.8
Spire Missouri |  |  |
Goodwill [Line Items] |  |  |
Accruals for capital expenditures | 28.9 | 14.8 | $ 9.6
Goodwill [Roll Forward] |  |  |
Beginning balance | 210.2 |  |
Ending balance | $ 210.2 | $ 210.2 |

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