BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NATURE OF BUSINESS (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) segment item	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
NATURE OF BUSINESS
Number of tankers	30
Number of operating segments | segment	1
Number of pool agreement terminations effective	0
Period for termination of pool agreements	90 days
Period to extend or reduce notice for termination of pool agreements	30 days
Net income (loss) | $	$ (35,203)	$ 26,864
Accumulated deficit | $	$ 299,859		$ 264,656
VLCCs
NATURE OF BUSINESS
Number of tankers	21
Suezmax tankers
NATURE OF BUSINESS
Number of tankers	6
Aframax
NATURE OF BUSINESS
Number of tankers	1
Panamax tankers
NATURE OF BUSINESS
Number of tankers	2