LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Components of Convertible Debt Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Amortization of debt discount and issuance costs	$ 7,022	$ 3,085	$ 541
Total interest expense	30,875	21,487	$ 12,667
Convertible debt
Debt Instrument [Line Items]
Coupon interest	11,500	4,823
Amortization of debt discount and issuance costs	5,886	2,310
Total interest expense	$ 17,386	$ 7,133
Effective interest rate	9.12%	9.12%