LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Detail of Convertible Debt Carrying Amount (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total		$ 376,958	$ 387,749
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		(6,088)	(6,820)
Unamortized discount		(30,243)	(33,882)
Net carrying amount of the liability component		193,669	189,298
Embedded derivative liability		0	20,594	[2]
Total	[3],[4]	$ 193,669	$ 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 condensed 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 September 30, 2018.

[3] Comprised of principal amount outstanding plus embedded derivative liability (if any), less unamortized discount and issuance costs. See Convertible Debt section below for additional information.

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