REVENUE RECOGNITION - Adjustments Upon Adoption of ASC 606 (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	$ 9,856	$ 5,732	$ 736
Deferred revenue - current	(4,591)	(2,658)	(4,424)
Long-term deferred revenue	(30,203)	(10,730)	(22,303)
Accumulated deficit	98,521	153,281	171,616
Calculated under revenue guidance in effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	883		736
Deferred revenue - current	(14,631)		(4,424)
Long-term deferred revenue	(100,329)		(22,303)
Accumulated deficit	193,850		$ 171,616
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	(8,973)	4,996
Deferred revenue - current	(10,040)	1,766
Long-term deferred revenue	(70,126)	11,573
Accumulated deficit	$ 95,329	(18,335)
Adjustment for Fixed Fee License Revenue | Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue - current		1,766
Long-term deferred revenue		11,573
Accumulated deficit		(13,339)
Elimination of Quarter-Lag Per-Unit Royalties | Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets		4,996
Accumulated deficit		$ (4,996)