LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Components of Convertible Debt Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt Instrument [Line Items]
Amortization of debt discount and issuance costs			$ 3,743	$ 3,416
Total interest expense	$ 7,586	$ 7,631	15,011	15,228
Convertible debt
Debt Instrument [Line Items]
Coupon interest	2,875	2,875	5,750	5,750
Amortization of debt discount and issuance costs	1,585	1,450	3,146	2,878
Total interest expense	$ 4,460	$ 4,325	$ 8,896	$ 8,628
Effective interest rate	9.12%	9.12%	9.12%	9.12%

Amount of amortization expense attributable to debt discount (premium) and debt issuance costs.

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-Name Accounting Standards Codification
-Topic 230
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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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