BASIS OF PRESENTATION - Accumulated deficit (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
BASIS OF PRESENTATION
Accumulated deficit	$ (274,724)	$ (285,200)

The cumulative amount of the reporting entity's undistributed earnings or deficit.

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