LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Long-Term Debt Balances (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total long-term debt and capital lease obligations		$ 357,383	$ 387,749
Less: current portion of long-term debt and capital lease obligations		569	0
Total long-term debt and capital lease obligations, less current portion		356,814	387,749
Credit Facility
Debt Instrument [Line Items]
Total long-term debt and capital lease obligations		156,843	177,857
Convertible debt
Debt Instrument [Line Items]
Total long-term debt and capital lease obligations	[1]	195,184	209,892
Capital lease obligations
Debt Instrument [Line Items]
Total long-term debt and capital lease obligations		$ 5,356	$ 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.