LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Detail of Convertible Debt Carrying Amount (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Carrying amount of the equity component, net of issuance costs	[1]	$ 13,912	$ 13,912
Convertible debt
Debt Instrument [Line Items]
Principal		230,000	230,000
Unamortized issuance costs		(5,572)	(5,834)
Unamortized discount		(27,682)	(28,982)
Net carrying amount of the liability component1	[2]	$ 196,746	$ 195,184

[1] Relates to the portion of the Notes accounted for under ASC 470-20 (defined below) and is included in the “Additional paid-in capital” line of the condensed consolidated balance sheets.

[2] Included in the “Long-term debt” line of the condensed consolidated balance sheets.