LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Components of Convertible Debt Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt Instrument [Line Items]
Amortization of debt discount and issuance costs	$ 1,860	$ 1,668
Total interest expense	7,425	7,597
Convertible debt
Debt Instrument [Line Items]
Coupon interest	2,875	2,875
Amortization of debt discount and issuance costs	1,561	1,428
Total interest expense	$ 4,436	$ 4,303
Effective interest rate	9.12%	9.12%