LONG-TERM DEBT, LETTERS OF CREDIT AND DERIVATIVES - Long-Term Debt Balances (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		$ 345,717	$ 357,383
Less: current portion of long-term debt and finance lease obligations		4,033	569
Total long-term debt and finance lease obligations, less current portion		341,684	356,814
Total Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		140,365	156,843
Convertible debt
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations	[1]	199,963	195,184
Finance lease obligations
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		5,389	5,356
Credit Facility revolver | Total Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		90,674	156,843
Credit Facility term loan | Total Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations	[2]	$ 49,691	$ 0

[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.