LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Detail of Convertible Debt Carrying Amount (Details) - USD ($) $ in Thousands		Dec. 31, 2017		Jul. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal		$ 407,857
Total		387,749			$ 366,911
Carrying amount of the equity component, net of issuance costs	[1]	13,912
Convertible Debt
Debt Instrument [Line Items]
Principal		230,000
Unamortized issuance costs		(6,820)
Unamortized discount		(33,882)		$ (35,800)
Net carrying amount of the liability component		189,298
Embedded derivative liability		20,594		$ 21,400
Total		$ 209,892	[2],[3]		$ 0

[1] Included in the Additional paid-in capital line of the consolidated balance sheet.

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

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