REVENUE RECOGNITION - Impact of Adoption of ASC 606 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenues:
Royalty and license	$ 5,992		$ 91,327
Development, services, and other	152		233
Total revenues	6,144		91,560
Operating Expenses	14,439		29,750
Operating income (loss)	(8,295)	$ (14,911)	61,810	$ (27,763)
Interest and other income	375	165	606	304
Income (loss) before provision for income taxes	(7,920)	(14,746)	62,416	(27,459)
Benefit (provision) for income taxes	162	(99)	(291)	(251)
Net income (loss)	$ (7,758)	$ (14,845)	$ 62,125	$ (27,710)
Basic net loss per share (in dollars per share)	$ (0.25)	$ (0.51)	$ 2.06	$ (0.95)
Diluted net loss per share (in dollars per share)	$ (0.25)	$ (0.51)	$ 2.00	$ (0.95)
Balance Sheets
Prepaid expenses and other current assets	$ 5,821		$ 5,821		$ 5,732	$ 736
Other assets	4,421		4,421			344
Deferred revenue - current	(4,769)		(4,769)		(2,658)	(4,424)
Long-term deferred revenue	(32,536)		(32,536)		(10,730)	(22,303)
Accumulated deficit	91,156		91,156		153,281	171,616
Fixed fee license revenue
Revenues:
Royalty and license	1,881		77,637
Per-unit royalty revenue
Revenues:
Royalty and license	4,111		13,690
Calculated under revenue guidance in effect before Topic 606
Revenues:
Royalty and license	8,517	$ 6,785	17,161	$ 15,791
Development, services, and other	152	245	233	463
Total revenues	8,669	7,030	17,394	16,254
Operating Expenses	14,439	21,941	29,750	44,017
Operating income (loss)	(5,770)	(14,911)	(12,356)	(27,763)
Interest and other income	375	165	606	304
Income (loss) before provision for income taxes	(5,395)	(14,746)	(11,750)	(27,459)
Benefit (provision) for income taxes	162	(99)	(291)	(251)
Net income (loss)	$ (5,233)	$ (14,845)	$ (12,041)	$ (27,710)
Basic net loss per share (in dollars per share)	$ (0.17)	$ (0.51)	$ (0.40)	$ (0.95)
Diluted net loss per share (in dollars per share)	$ (0.17)	$ (0.51)	$ (0.40)	$ (0.95)
Balance Sheets
Prepaid expenses and other current assets	$ 698		$ 698			736
Other assets	296		296			344
Deferred revenue - current	(14,364)		(14,364)			(4,424)
Long-term deferred revenue	(106,387)		(106,387)			(22,303)
Accumulated deficit	183,657		183,657			$ 171,616
Calculated under revenue guidance in effect before Topic 606 | Fixed fee license revenue
Revenues:
Royalty and license	4,312	$ 1,765	7,728	$ 4,275
Calculated under revenue guidance in effect before Topic 606 | Per-unit royalty revenue
Revenues:
Royalty and license	4,205	$ 5,020	9,433	$ 11,516
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606
Revenues:
Royalty and license	2,525		(74,166)
Development, services, and other	0		0
Total revenues	2,525		(74,166)
Operating income (loss)	2,525		(74,166)
Interest and other income	0		0
Income (loss) before provision for income taxes	2,525		(74,166)
Benefit (provision) for income taxes	0		0
Net income (loss)	$ 2,525		$ (74,166)
Basic net loss per share (in dollars per share)	$ 0.08		$ (2.46)
Diluted net loss per share (in dollars per share)	$ 0.08		$ (2.46)
Balance Sheets
Prepaid expenses and other current assets	$ (5,123)		$ (5,123)		4,996
Other assets	(4,125)		(4,125)
Deferred revenue - current	(9,595)		(9,595)		1,766
Long-term deferred revenue	(73,851)		(73,851)		11,573
Accumulated deficit	92,501		92,501		(18,335)
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606 | Fixed fee license revenue
Revenues:
Royalty and license	2,431		(69,909)
Balance Sheets
Deferred revenue - current					1,766
Long-term deferred revenue					11,573
Accumulated deficit					(13,339)
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606 | Per-unit royalty revenue
Revenues:
Royalty and license	$ 94		$ (4,257)
Balance Sheets
Prepaid expenses and other current assets					4,996
Accumulated deficit					$ (4,996)