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STOCK-BASED COMPENSATION PLANS (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
STOCK-BASED COMPENSATION PLANS    
Recognized employee stock-based compensation expense $ 8,900,000 $ 9,600,000
Employee Stock Purchase Plan    
STOCK-BASED COMPENSATION PLANS    
Shares available for grant 1,161,220  
Weighted-Average Grant Date Fair Value    
Shares available under the plan 3,600,000  
Offering period for the plan (in months) 3 months  
Period of interval between occurrence of purchases (in months) 3 months  
IRS regulated cap for payroll deduction to purchase common stock $ 25,000  
Purchase price expressed as a percentage of fair market value of common stock (as a percent) 90.00%  
Shares issued under ESPP 237,726  
Shares available for purchase under ESPP 1,161,220  
Minimum | Employee Stock Purchase Plan    
Weighted-Average Grant Date Fair Value    
Eligible service period of employees per week to participate in the plan (in hours) 20 hours  
Eligible service period of employees in a calendar year to participate in the plan (in months) 5 months  
Percentage of employees' eligible earnings as payroll deduction to purchase common stock 1.00%  
Maximum | Employee Stock Purchase Plan    
Weighted-Average Grant Date Fair Value    
Percentage of employees' eligible earnings as payroll deduction to purchase common stock 10.00%  
2007 Equity Incentive Plan    
STOCK-BASED COMPENSATION PLANS    
Shares available for grant 3,455,800  
Weighted-Average Grant Date Fair Value    
Total fair value RSUs vested (in dollars) $ 9,700,000 $ 10,100,000
Unrecognized compensation cost related to the RSUs $ 8,800,000  
2007 Equity Incentive Plan | Maximum    
STOCK-BASED COMPENSATION PLANS    
Expiration period (in years) 10 years  
2007 Equity Incentive Plan | Restricted stock units    
STOCK-BASED COMPENSATION PLANS    
Percentage of relevant period has elapsed for pro rata vesting of the awards 50.00%  
RSU    
Non-vested at the beginning of the period (in shares) 5,575,000 4,757,000
Granted (in shares) 4,249,000 3,838,000
Vested (in shares) (2,634,000) (2,564,000)
Forfeited (in shares) (324,000) (456,000)
Non-vested at the end of the period (in shares) 6,866,000 5,575,000
Weighted-Average Grant Date Fair Value    
Non-vested at the beginning of the period (in dollars per share) $ 3.22 $ 3.98
Granted (in dollars per share) 2.01 2.93
Vested (in dollars per share) 3.68 3.94
Forfeited (in dollars per share) 3.00 4.08
Non-vested at the end of the period (in dollars per share) $ 2.31 $ 3.22
Weighted-average period to recognize unrecognized compensation cost 2 years 1 month 6 days  
2007 Equity Incentive Plan | Restricted stock units | Awards Vesting On Employment Through The Fourth Anniversary Of Grant    
STOCK-BASED COMPENSATION PLANS    
Award vesting percentage 100.00%  
2007 Equity Incentive Plan | Restricted stock units | Awards Vesting On Change In Control    
STOCK-BASED COMPENSATION PLANS    
Award vesting percentage 100.00%  
Incentive Plan Amendment    
STOCK-BASED COMPENSATION PLANS    
Additional shares authorized 5,500,000