LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Components of Convertible Debt Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Amortization of debt discount and issuance costs	$ 7,695	$ 7,022	$ 3,085
Total interest expense	29,713	30,875	$ 21,487
Convertible debt
Debt Instrument [Line Items]
Coupon interest	11,500	11,500
Amortization of debt discount and issuance costs	6,435	5,886
Total interest expense	$ 17,935	$ 17,386
Effective interest rate	9.12%	9.12%