LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Detail of Convertible Debt Carrying Amount (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018	Jul. 31, 2017
Debt Instrument [Line Items]
Principal		$ 353,876
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		(4,756)	(5,834)
Unamortized discount		(23,625)	(28,982)	$ (35,800)
Net carrying amount of the liability component	[2]	$ 201,619	$ 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 consolidated balance sheets.

[2] Included in the “Long-term debt and finance lease obligations” line of the consolidated balance sheets.