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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - SALE-LEASEBACKS (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Sale Leaseback Transaction [Line Items] |  |  |  |  |
Proceeds from sale-leaseback transaction |  |  | $ 7,820 | $ 9,587 | $ 7,107
Proceeds from sale-leaseback transaction - solar |  |  | $ 71,538 | 32,901 | 0
Natural Gas Distribution |  |  |  |  |
Sale Leaseback Transaction [Line Items] |  |  |  |  |
Proceeds from sale-leaseback transaction | $ 7,800 | $ 9,600 |  |  | $ 7,100
Minimum | Natural Gas Distribution |  |  |  |  |
Sale Leaseback Transaction [Line Items] |  |  |  |  |
Term of lease |  |  | 7 years |  |
Maximum | Natural Gas Distribution |  |  |  |  |
Sale Leaseback Transaction [Line Items] |  |  |  |  |
Term of lease |  |  | 11 years |  |
Clean Energy Ventures |  |  |  |  |
Sale Leaseback Transaction [Line Items] |  |  |  |  |
Proceeds from sale-leaseback transaction - solar |  |  | $ 71,538 | $ 32,901 |
Clean Energy Ventures | Minimum |  |  |  |  |
Sale Leaseback Transaction [Line Items] |  |  |  |  |
Term of lease |  |  | 6 years |  |
Clean Energy Ventures | Maximum |  |  |  |  |
Sale Leaseback Transaction [Line Items] |  |  |  |  |
Term of lease |  |  | 15 years |  |

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- Definition Amount of cash inflow after closing and debt issuance costs received by a seller-lessee in a sale-leaseback recognized in financing activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 55 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=82846721&loc=d3e51831-112757 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 55 -Paragraph 51 -URI http://asc.fasb.org/extlink&oid=82846721&loc=d3e51840-112757 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 55 -Paragraph 52 -URI http://asc.fasb.org/extlink&oid=82846721&loc=d3e51843-112757 + Details
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