LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Long-Term Debt Balances (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total long-term debt		$ 376,958	$ 387,749
Less: current portion of long-term debt		0	0
Total long-term debt, less current portion		376,958	387,749
Credit Facility
Debt Instrument [Line Items]
Total long-term debt		183,289	177,857
Convertible debt
Debt Instrument [Line Items]
Total long-term debt	[1],[2]	$ 193,669	$ 209,892

[1] 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.

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