LONG-TERM DEBT, DERIVATIVES AND LETTERS OF CREDIT - Long-Term Debt Balances (Details) - USD ($) $ in Thousands	Sep. 30, 2017		Dec. 31, 2016
Debt Instrument [Line Items]
Total long-term debt	$ 366,026		$ 366,911
Less: current portion of long-term debt	0		20,000
Total long-term debt, less current portion	366,026		346,911
Credit Facility
Debt Instrument [Line Items]
Total long-term debt	163,042		366,911
Convertible debt
Debt Instrument [Line Items]
Total long-term debt	$ 202,984	[1]	$ 0

[1] Included in the Long-term debt line of the condensed consolidated balance sheet.