REVENUE RECOGNITION - Adjustments Upon Adoption of ASC 606 (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	$ 7,802	$ 5,732	$ 736
Deferred revenue - current	(4,694)	(2,658)	(4,424)
Long-term deferred revenue	(31,407)	(10,730)	(22,303)
Accumulated deficit	95,379	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	723		736
Deferred revenue - current	(15,409)		(4,424)
Long-term deferred revenue	(103,568)		(22,303)
Accumulated deficit	188,814		$ 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	(7,079)	4,996
Deferred revenue - current	(10,715)	1,766
Long-term deferred revenue	(72,161)	11,573
Accumulated deficit	$ 93,435	(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)