STOCK INCENTIVE AND OTHER BENEFIT PLANS - Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 31,809,000	$ 17,383,000	$ 24,592,000
Reversal of stock based compensation expense		2,455,000
Total compensation expense related to unvested awards not yet recognized	$ 45,841,000
Quoted closing price of Company's common stock	$ 9.01
Cash proceeds, net of taxes from option exercises	$ 19,463,000	3,305,000	5,643,000
2007 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,832,000	1,360,000	927,000
Number of shares available for grant	2,003
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 19,942,000	10,466,000	12,807,000
Sales and marketing
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	3,904,000	1,700,000	2,795,000
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	7,963,000	5,217,000	8,990,000
Options to purchase common stock
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	6,068,000	7,043,000	6,386,000
Total compensation expense related to unvested awards not yet recognized	$ 5,050,000
Unrecognized compensation expense recognized over weighted average period	1 year 3 months 3 days
Total intrinsic value exercised	$ 23,991,000	7,806,000	10,957,000
Significant income tax benefit from stock option exercises	$ 0	0	0
Restricted stock units ("RSUs")
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Vesting period	4 years
Vesting percentage on or around the one-year anniversary	25.00%
Total stock-based compensation expense	$ 11,177,000	6,559,000	8,084,000
Total compensation expense related to unvested awards not yet recognized	$ 36,789,000
Unrecognized compensation expense recognized over weighted average period	3 years 2 months 12 days
Performance stock options ("PSOs")
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 12,732,000	$ 2,421,000	$ 9,195,000
Total compensation expense related to unvested awards not yet recognized	$ 325,000
Unrecognized compensation expense recognized over weighted average period	1 month 17 days
Performance stock units ("PSUs")
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 739,000
Total compensation expense related to unvested awards not yet recognized	$ 3,677,000
Unrecognized compensation expense recognized over weighted average period	6 months 7 days

GLU MOBILE INC 10-K