INCOME TAXES - Rollforward of deferred tax asset valuation allowance (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Asset Valuation Allowance [Roll Forward]
Beginning	$ (55,223)	$ (46,630)	$ (34,952)
Increase to allowance	(3,608)	(8,593)	(11,678)
Ending	$ (58,831)	$ (55,223)	$ (46,630)

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

Name:	us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant