LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Detail of Convertible Debt Carrying Amount (Details) - USD ($) $ in Thousands		Dec. 31, 2018		Dec. 31, 2017		Jul. 31, 2017
Debt Instrument [Line Items]
Principal		$ 386,843
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		(5,834)		(6,820)
Unamortized discount		(28,982)		(33,882)		$ (35,800)
Net carrying amount of the liability component		195,184		189,298
Embedded derivative liability		0	[2]	20,594	[2]	$ 21,400
Total	[3]	$ 195,184		$ 209,892

[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] The embedded derivative liability was reclassified to stockholders’ equity as of May 17, 2018 and is no longer marked to fair value each period, as discussed further below. It is excluded from the table above as of December 31, 2018.

[3] Included in the Long-term debt line of the consolidated balance sheets.