LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Long-Term Debt Balances (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		$ 363,770	$ 357,383
Less: current portion of long-term debt and finance lease obligations		279	569
Total long-term debt and finance lease obligations, less current portion		363,491	356,814
Credit Facility
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		159,987	156,843
Convertible debt
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations	[1]	198,330	195,184
Finance lease obligations
Debt Instrument [Line Items]
Total long-term debt and finance lease obligations		$ 5,453	$ 5,356

[1] Comprised of principal amount outstanding, less unamortized discount and issuance costs. See Convertible Debt section below for additional information.