SUPPLEMENTAL CASH FLOWS INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental cash flows disclosures
Interest paid	$ 3,966	$ 18,249	$ 20,531
Income taxes paid	0	3	1,295
Supplemental disclosure of non‑cash activities
Capital expenditures included in accounts payable	33,850	3,176	8,821
Conversion of preferred stock to common stock at Initial Public Offering	$ 162,511	$ 0	$ 0