LONG-TERM DEBT, LETTERS OF CREDIT AND DERIVATIVES - Long-Term Debt Balances (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		$ 350,658	$ 330,593
Less: current portion of long-term debt and finance lease obligations		5,296	5,294
Total long-term debt and finance lease obligations, less current portion		345,362	325,299
Finance lease obligations		5,289	5,363
Total Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		142,043	123,611
Convertible debt
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations	[1]	203,326	201,619
Credit Facility revolver | Total Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		93,513	73,876
Credit Facility term loan | Total Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations	[2]	$ 48,530	$ 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.