REVENUE - Effect of Adoption of ASC 606 Affect Prior year (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jul. 01, 2018	Jun. 30, 2018	Mar. 31, 2018
ASSETS
Prepaid expenses and other current assets	$ 2,878			$ 1,180	$ 929	$ 1,278
Other assets	1,922			1,974	720	414
LIABILITIES
Deferred revenue	1,322	$ 1,437		1,638	511	511
SHAREHOLDERS' EQUITY
Accumulated deficit	(253,859)			(232,372)	(232,748)	$ (231,052)
Calculated under Revenue Guidance in Effect before Topic 606
ASSETS
Prepaid expenses and other current assets	2,615				929
Other assets					720
LIABILITIES
Deferred revenue			$ 1,437		511
SHAREHOLDERS' EQUITY
Accumulated deficit	(254,427)				$ (232,748)
Adjustment | ASC 606
ASSETS
Prepaid expenses and other current assets	(263)			251
Other assets				1,254
LIABILITIES
Deferred revenue		$ 0		1,127
SHAREHOLDERS' EQUITY
Accumulated deficit	$ (568)			$ 376