LONG-TERM DEBT, LETTERS OF CREDIT AND DERIVATIVES - Long-Term Debt Balances (Details) - USD ($) $ in Thousands		Jun. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		$ 326,410	$ 330,593
Less: current portion of long-term debt and finance lease obligations		5,316	5,294
Total long-term debt and finance lease obligations, less current portion		321,094	325,299
Finance lease obligations		5,269	5,363
Unamortized discount and issuance costs		176	265
Total Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		116,083	123,611
Convertible debt
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations	[1]	205,058	201,619
Credit Facility revolver | Total Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		68,759	73,876
Credit Facility term loan | Total Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations	[2]	$ 47,324	$ 49,735

[1] Comprised of principal amount outstanding, 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 of $176 thousand as of June 30, 2020 and $265 thousand as of December 31, 2019.