REVENUE RECOGNITION - Impact of Adoption of ASC 606 (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Revenues:
Revenue									$ 110,979	$ 35,013	$ 57,086
Total costs and operating expenses									57,878
Operating income (loss)	$ (3,546)	$ (5,163)	$ (8,295)	$ 70,105	$ (12,216)	$ (5,443)	$ (14,911)	$ (12,852)	53,101	(45,422)	(15,263)
Interest and other income									1,634
Income (loss) from continuing operations before provision for income taxes	(3,063)	(4,618)	(7,920)	70,336	(12,109)	(5,243)	(14,746)	(12,713)	54,735	(44,811)	(14,509)
Provision for income taxes from continuing operations	(79)	(22)	162	(453)	(185)	(44)	(99)	(152)	(392)	(480)	(25,521)
Income (loss) from continuing operations									54,343	(45,291)	(40,030)
Income from discontinued operations, net of tax									0	0	649
Net income (loss)	$ (3,142)	$ (4,640)	$ (7,758)	$ 69,883	$ (12,294)	$ (5,287)	$ (14,845)	$ (12,865)	$ 54,343	$ (45,291)	$ (39,381)
Basic net income (loss) per share:
Continuing operations (in dollars per share)									$ 1.78	$ (1.55)	$ (1.39)
Discontinued operations (in dollars per share)									0.00	0.00	0.02
Total (in dollars per share)	$ (0.10)	$ (0.15)	$ (0.25)	$ 2.35	$ (0.42)	$ (0.18)	$ (0.51)	$ (0.44)	$ 1.78	$ (1.55)	$ (1.37)
Shares used in calculating basic net income (loss) per share (in shares)	30,814	30,780	30,527	29,700	29,250	29,245	29,193	29,024	30,459	29,179	28,759
Diluted net income (loss) per share:
Continuing operations (in dollars per share)									$ 1.73	$ (1.55)	$ (1.39)
Discontinued operations (in dollars per share)									0.00	0.00	0.02
Total (in dollars per shares)	$ (0.10)	$ (0.15)	$ (0.25)	$ 2.29	$ (0.42)	$ (0.18)	$ (0.51)	$ (0.44)	$ 1.73	$ (1.55)	$ (1.37)
Shares used in calculating diluted net income (loss) per share (in shares)	30,814	30,780	30,527	30,566	29,250	29,245	29,193	29,024	31,407	29,179	28,759
Balance Sheets
Prepaid expenses and other current assets	$ 9,856				$ 736				$ 9,856	$ 736		$ 5,732
Other assets, net	7,532				344				7,532	344
Other current liabilities	(3,194)				(3,896)				(3,194)	(3,896)
Deferred revenue - current	(4,591)				(4,424)				(4,591)	(4,424)		(2,658)
Long-term deferred revenue	(30,203)				(22,303)				(30,203)	(22,303)		(10,730)
Accumulated deficit	98,521				171,616				98,521	171,616		153,281
Royalty and license
Revenues:
Revenue									110,557	34,089	$ 56,030
Fixed fee license revenue
Revenues:
Revenue									83,573	12,575	30,389
Per-unit royalty revenue
Revenues:
Revenue									26,984	21,514	25,641
Development, services, and other
Revenues:
Revenue									422	924	1,056
Calculated under revenue guidance in effect before Topic 606
Revenues:
Revenue									34,403	35,013	57,086
Total costs and operating expenses									57,878	80,435	72,349
Operating income (loss)									(23,475)	(45,422)	(15,263)
Interest and other income									1,634	611	754
Income (loss) from continuing operations before provision for income taxes									(21,841)	(44,811)	(14,509)
Provision for income taxes from continuing operations									(392)	(480)	(25,521)
Income (loss) from continuing operations									(22,233)	(45,291)	(40,030)
Income from discontinued operations, net of tax									0	0	649
Net income (loss)									$ (22,233)	$ (45,291)	$ (39,381)
Basic net income (loss) per share:
Continuing operations (in dollars per share)									$ (0.73)	$ (1.55)	$ (1.39)
Discontinued operations (in dollars per share)									0.00	0.00	0.02
Total (in dollars per share)									$ (0.73)	$ (1.55)	$ (1.37)
Shares used in calculating basic net income (loss) per share (in shares)									30,459	29,179	28,759
Diluted net income (loss) per share:
Continuing operations (in dollars per share)									$ (0.73)	$ (1.55)	$ (1.39)
Discontinued operations (in dollars per share)									0.00	0.00	0.02
Total (in dollars per shares)									$ (0.73)	$ (1.55)	$ (1.37)
Shares used in calculating diluted net income (loss) per share (in shares)									30,459	29,179	28,759
Balance Sheets
Prepaid expenses and other current assets	883				736				$ 883	$ 736
Other assets, net	301				344				301	344
Other current liabilities	(2,974)				(3,896)				(2,974)	(3,896)
Deferred revenue - current	(14,631)				(4,424)				(14,631)	(4,424)
Long-term deferred revenue	(100,329)				(22,303)				(100,329)	(22,303)
Accumulated deficit	193,850				$ 171,616				193,850	171,616
Calculated under revenue guidance in effect before Topic 606 | Royalty and license
Revenues:
Revenue									33,981	34,089	$ 56,030
Calculated under revenue guidance in effect before Topic 606 | Fixed fee license revenue
Revenues:
Revenue									15,479	12,575	30,389
Calculated under revenue guidance in effect before Topic 606 | Per-unit royalty revenue
Revenues:
Revenue									18,502	21,514	25,641
Calculated under revenue guidance in effect before Topic 606 | Development, services, and other
Revenues:
Revenue									422	$ 924	$ 1,056
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606
Revenues:
Revenue									(76,576)
Operating income (loss)									(76,576)
Interest and other income									0
Income (loss) from continuing operations before provision for income taxes									(76,576)
Provision for income taxes from continuing operations									0
Income (loss) from continuing operations									(76,576)
Income from discontinued operations, net of tax									0
Net income (loss)									$ (76,576)
Basic net income (loss) per share:
Continuing operations (in dollars per share)									$ (2.51)
Discontinued operations (in dollars per share)									0.00
Total (in dollars per share)									$ (2.51)
Shares used in calculating basic net income (loss) per share (in shares)									30,459
Diluted net income (loss) per share:
Continuing operations (in dollars per share)									$ (2.51)
Discontinued operations (in dollars per share)									0.00
Total (in dollars per shares)									$ (2.51)
Shares used in calculating diluted net income (loss) per share (in shares)									30,459
Balance Sheets
Prepaid expenses and other current assets	(8,973)								$ (8,973)			4,996
Other assets, net	(7,231)								(7,231)
Other current liabilities	220								220
Deferred revenue - current	(10,040)								(10,040)			1,766
Long-term deferred revenue	(70,126)								(70,126)			11,573
Accumulated deficit	$ 95,329								95,329			(18,335)
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606 | Royalty and license
Revenues:
Revenue									(76,576)
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606 | Fixed fee license revenue
Revenues:
Revenue									(68,094)
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									$ (8,482)
Balance Sheets
Prepaid expenses and other current assets												4,996
Accumulated deficit												$ (4,996)