LONG-TERM DEBT, LETTERS OF CREDIT AND DERIVATIVES - Long-Term Debt Balances (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Total debt and finance lease obligations		$ 344,195	$ 330,593
Less: current portion of long-term debt and finance lease obligations		5,323	5,294
Total long-term debt and finance lease obligations, less current portion		338,872	325,299
Finance lease obligations		5,207	5,363
Unamortized discount and issuance costs		132	265
Total Credit Facility
Debt Instrument [Line Items]
Total debt and finance lease obligations		132,145	123,611
Convertible debt
Debt Instrument [Line Items]
Total debt and finance lease obligations	[1]	206,843	201,619
Credit Facility revolver | Total Credit Facility
Debt Instrument [Line Items]
Total debt and finance lease obligations		86,028	73,876
Credit Facility term loan | Total Credit Facility
Debt Instrument [Line Items]
Total debt and finance lease obligations	[2]	$ 46,117	$ 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 $132 thousand as of September 30, 2020 and $265 thousand as of December 31, 2019.