LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Detail of Convertible Debt Carrying Amount (Details) - USD ($) $ in Thousands		Sep. 30, 2017		Jul. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Embedded derivative liability				$ 21,400
Total		$ 366,026			$ 366,911
Carrying amount of the equity component, net of issuance costs	[1]	13,955
Convertible debt
Debt Instrument [Line Items]
Principal		230,000
Unamortized issuance costs		(7,101)
Unamortized discount		(35,035)		$ (35,800)
Net carrying amount of the liability component		187,864
Embedded derivative liability		15,120
Total		$ 202,984	[2]		$ 0

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

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