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LONG-TERM DEBT - Tax-Exempt Offerings (Details) - Unsecured Debt - USD ($) | 3 Months Ended |
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Jun. 30, 2018 | Mar. 31, 2017
Finance Authority Of Maine Solid Waste Disposal Revenue Bonds Series 2015R-2 |  |
Debt Instrument [Line Items] |  |
Aggregate principal amount | $ 15,000,000 |
Stated interest rate | 4.375% |
VEDA Bonds |  |
Debt Instrument [Line Items] |  |
Aggregate principal amount | $ 16,000,000 |
Stated interest rate | 4.75% |
Debt instrument, interest rate during period | 4.625% |
Revenue Bonds Series Two Thousand Five Rone Due Two Thousand Twenty Five |  |
Debt Instrument [Line Items] |  |
Aggregate principal amount |  | $ 3,600,000
Revenue Bonds Series Two Thousand Five Rtwo Due Two Thousand Twenty Five |  |
Debt Instrument [Line Items] |  |
Aggregate principal amount |  | 21,400,000
Finance Authority of Maine Solid Waste Disposal Revenue Bonds Series 2005R-3 (FAME Bonds 2005R-3) |  |
Debt Instrument [Line Items] |  |
Aggregate principal amount |  | $ 25,000,000
Stated interest rate |  | 5.25%

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