STOCKHOLDERS' EQUITY - EQUITY PLANS - SCHEDULE OF AGGREGATE STOCK OPTION AND AWARD ACTIVITY (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Shares Available For Grant
Balance at the beginning of the period (in shares)	7,257,747
Additional shares authorized (in shares)	18,300,000
Stock options granted (in shares)	(9,562,106)
Awards granted (in shares)	(20,783)
Stock options cancelled/forfeited/expired (in shares)	2,788,535
Balance at the end of the period (in shares)	18,763,393
Number of Shares - Options Outstanding
Balance at the beginning of the period (in shares)	43,663,691
Stock options granted (in shares)	9,562,106
Stock options exercised (in shares)	(451,853)
Stock options cancelled/forfeited/expired (in shares)	(2,821,535)
Balance at the end of the period (in shares)	49,952,409
Stock options exercisable at the end of the period (in shares)	30,459,136	25,721,508	19,915,713
Stock options fully vested and expected to vest at the end of the period (in shares)	49,085,603
Weighted Average Exercise Price Per Share - Options Outstanding
Balance at the beginning of the period (in dollars per share)	$ 2.12
Stock options granted (in dollars per share)	1.81
Stock options exercised (in dollars per share)	1.23
Stock options cancelled/forfeited/expired (in dollars per share)	2.31
Balance at the end of the period (in dollars per share)	2.06
Stock options exercisable at the end of the period (in dollars per share)	2.35	$ 2.54	$ 2.86
Stock options fully vested and expected to vest at the end of the period (in dollars per share)	$ 2.06
Weight Average Remaining Contractual Life (in years) - Options Outstanding
Balance at the end of the period	6 years 1 month 2 days
Stock options exercisable at the end of the period	4 years 9 months 25 days
Stock options fully vested and expected to vest at the end of the period	6 years 21 days
Aggregate Intrinsic Value - Options Outstanding
Balance at the end of the period (in dollars)	$ 890
Stock options exercisable at the end of the period (in dollars)	547
Stock options fully vested and expected to vest at the end of the period (in dollars)	$ 878