REVENUE RECOGNITION - Impact of Adoption of ASC 606 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenues:
Revenue	$ 8,552	$ 11,863	$ 100,112	$ 28,117
Operating expenses	13,715		43,465
Operating income (loss)	(5,163)	(5,443)	56,647	(33,206)
Interest and other income	545	200	1,151	504
Income (loss) before provision for income taxes	(4,618)	(5,243)	57,798	(32,702)
Provision for income taxes	(22)	(44)	(313)	(295)
Net income (loss)	$ (4,640)	$ (5,287)	$ 57,485	$ (32,997)
Basic net loss per share (in dollars per share)	$ (0.15)	$ (0.18)	$ 1.89	$ (1.13)
Diluted net loss per share (in dollars per share)	$ (0.15)	$ (0.18)	$ 1.83	$ (1.13)
Balance Sheets
Prepaid expenses and other current assets	$ 7,802		$ 7,802		$ 5,732	$ 736
Other assets	4,254		4,254			344
Other current liabilities	(5,703)		(5,703)			(3,896)
Deferred revenue - current	(4,694)		(4,694)		(2,658)	(4,424)
Long-term deferred revenue	(31,407)		(31,407)		(10,730)	(22,303)
Accumulated deficit	95,379		95,379		153,281	171,616
Total royalty and license revenue
Revenues:
Revenue	8,462	$ 11,636	99,789	$ 27,427
Fixed fee license revenue
Revenues:
Revenue	4,317	6,881	81,955	11,156
Per-unit royalty revenue
Revenues:
Revenue	4,145	4,755	17,834	16,271
Development, services, and other revenue
Revenues:
Revenue	90	227	323	690
Calculated under revenue guidance in effect before Topic 606
Revenues:
Revenue	8,032	11,863	25,429	28,117
Operating expenses	13,715	17,306	43,465	61,323
Operating income (loss)	(5,683)	(5,443)	(18,036)	(33,206)
Interest and other income	545	200	1,151	504
Income (loss) before provision for income taxes	(5,138)	(5,243)	(16,885)	(32,702)
Provision for income taxes	(22)	(44)	(313)	(295)
Net income (loss)	$ (5,160)	$ (5,287)	$ (17,198)	$ (32,997)
Basic net loss per share (in dollars per share)	$ (0.17)	$ (0.18)	$ (0.57)	$ (1.13)
Diluted net loss per share (in dollars per share)	$ (0.17)	$ (0.18)	$ (0.57)	$ (1.13)
Balance Sheets
Prepaid expenses and other current assets	$ 723		$ 723			736
Other assets	291		291			344
Other current liabilities	(5,523)		(5,523)			(3,896)
Deferred revenue - current	(15,409)		(15,409)			(4,424)
Long-term deferred revenue	(103,568)		(103,568)			(22,303)
Accumulated deficit	188,814		188,814			$ 171,616
Calculated under revenue guidance in effect before Topic 606 | Total royalty and license revenue
Revenues:
Revenue	7,942	$ 11,636	25,106	$ 27,427
Calculated under revenue guidance in effect before Topic 606 | Fixed fee license revenue
Revenues:
Revenue	3,818	6,881	11,546	11,156
Calculated under revenue guidance in effect before Topic 606 | Per-unit royalty revenue
Revenues:
Revenue	4,124	4,755	13,560	16,271
Calculated under revenue guidance in effect before Topic 606 | Development, services, and other revenue
Revenues:
Revenue	90	$ 227	323	$ 690
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606
Revenues:
Revenue	(520)		(74,683)
Operating income (loss)	(520)		(74,683)
Interest and other income	0		0
Income (loss) before provision for income taxes	(520)		(74,683)
Provision for income taxes	0		0
Net income (loss)	$ (520)		$ (74,683)
Basic net loss per share (in dollars per share)	$ (0.02)		$ (2.46)
Diluted net loss per share (in dollars per share)	$ (0.02)		$ (2.46)
Balance Sheets
Prepaid expenses and other current assets	$ (7,079)		$ (7,079)		4,996
Other assets	(3,963)		(3,963)
Other current liabilities	180		180
Deferred revenue - current	(10,715)		(10,715)		1,766
Long-term deferred revenue	(72,161)		(72,161)		11,573
Accumulated deficit	93,435		93,435		(18,335)
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606 | Total royalty and license revenue
Revenues:
Revenue	(520)		(74,683)
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606 | Fixed fee license revenue
Revenues:
Revenue	(499)		(70,409)
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:
Revenue	(21)		(4,274)
Balance Sheets
Prepaid expenses and other current assets					4,996
Accumulated deficit					$ (4,996)
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606 | Development, services, and other revenue
Revenues:
Revenue	$ 0		$ 0