LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Long-Term Debt Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
Debt Instrument [Line Items]
Total long-term debt	$ 387,749		$ 366,911
Current-portion of long term debt	0		20,000
Total long-term debt, less current portion	387,749		346,911
Credit Facility
Debt Instrument [Line Items]
Total long-term debt	177,857		366,911
Convertible Debt
Debt Instrument [Line Items]
Total long-term debt	$ 209,892	[1],[2]	$ 0

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

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