LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Long-Term Debt Balances (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		$ 330,593	$ 357,383
Finance lease obligations		5,363
Less: current portion of long-term debt and finance lease obligations		5,294	569
Total long-term debt and finance lease obligations, less current portion		325,299	356,814
Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		123,611	156,843
Convertible debt
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations	[1]	201,619	195,184
Finance lease obligations
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations			5,356
Credit Facility revolver | Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		73,876	156,843
Credit Facility term loan | Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations	[2]	$ 49,735	$ 0

[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] Comprised of principal amount outstanding, less unamortized discount and issuance costs.