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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 12, 2016 USD ($) | Dec. 31, 2016 USD ($) reporting_unit | Dec. 31, 2015 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($)
Related Party Transaction [Line Items] |  |  |  |  |
Proceeds from settlement for acquisition of EnergySouth |  | $ 3.8 | $ 0.0 |  |
Number of operating segments | reporting_unit |  | 2 |  |  |
Finance receivables, net |  | $ 11.8 | 11.2 | $ 11.8 |
Laclede Gas |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Accrued unbilled revenues |  | 103.5 | 74.6 | 26.1 |
Accruals for capital expenditures |  | 6.8 | 4.6 | 14.8 |
Alagasco |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Accrued unbilled revenues |  | 22.0 | 16.4 | 5.9 |
Accruals for capital expenditures |  | $ 5.6 | 3.1 |  | $ 6.8
Threshold period past due for collection |  | 90 days |  |  |
Threshold period past due for write-off of financing receivable |  | 12 months |  |  |
Alagasco | Financing Receivables, Equal to Greater than 90 Days Past Due |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Financing receivable 90 days past due |  | $ 0.4 | $ 0.4 | $ 0.4 |
EnergySouth |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Proceeds from settlement for acquisition of EnergySouth | $ 3.8 |  |  |  |

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