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STOCK-BASED COMPENSATION Narrative (Details)
3 Months Ended 12 Months Ended
Jan. 01, 2020
shares
May 18, 2017
shares
Mar. 01, 2017
shares
Mar. 29, 2012
shares
Mar. 31, 2019
$ / shares
shares
Dec. 31, 2019
USD ($)
purchase_period
$ / shares
shares
Dec. 31, 2018
$ / shares
shares
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based compensation, outstanding (in shares)           4,097,000 6,772,000 8,426,000 8,730,000
Total unrecognized compensation cost | $           $ 31,500,000      
Weighted-average recognition period for unrecognized compensation cost           2 years 3 months 18 days      
Common stock to be purchased in exchange for options (in shares)           102,000 521,000 4,379,000  
Minimum                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Performance target percentage         0.00%        
Maximum                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Performance target percentage         200.00%        
Stock options                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Common stock to be purchased in exchange for options (in shares)     2,362,470            
Performance shares                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Award granted (in shares)         1,000,000.0 1,052,000 1,477,000    
Weighted average estimated fair value of award (in usd per share) | $ / shares           $ 9.48 $ 4.65    
Performance shares without market conditions                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Weighted average estimated fair value of award (in usd per share) | $ / shares         $ 8.80        
Performance shares with market conditions                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Award granted (in shares)         500,000        
Weighted average estimated fair value of award (in usd per share) | $ / shares         $ 10.70        
Restricted stock                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Common stock to be purchased in exchange for options (in shares)     733,559            
Two Thousand Six Equity Incentive Plan                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based compensation, vesting period           4 years      
Share-based compensation, weighted-average remaining contractual term           10 years      
Share-based compensation, outstanding (in shares)           100,000      
Two Thousand and Eleven Equity Incentive Plan                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based compensation, vesting period           4 years      
Share-based compensation, shares authorized (in shares)           2,643,171      
Increase in common stock shares reserved for future issuance percentage           4.50%      
Share-based compensation, shares available for grant (in shares)           4,355,838      
Employee Stock Purchase Plan (ESPP)                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based compensation, shares authorized (in shares)       669,603          
Share-based compensation, shares available for grant (in shares)           936,020      
Share-based compensation, additional shares authorized (in shares)   400,000              
Share-based compensation, evergreen shares (in shares)   700,000   330,396          
Share-based compensation, increase in number of shares authorized       1.00%          
Employee stock purchase plan, number of interim purchase periods | purchase_period           4      
General duration of employee stock purchase plan           24 months      
Share-based compensation, maximum employee subscription rate           15.00%      
Share-based compensation, purchase price of common stock percent to fair market value           85.00%      
Share-based compensation, look-back feature (in years)           2 years      
IRS limitation for employees right to acquire class common stock under ESPP | $           $ 25,000      
Before August 1, 2012 | Two Thousand and Eleven Equity Incentive Plan                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based compensation, weighted-average remaining contractual term           10 years      
After August 1, 2012 | Two Thousand and Eleven Equity Incentive Plan                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based compensation, weighted-average remaining contractual term           7 years      
Subsequent Event | Two Thousand and Eleven Equity Incentive Plan                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based compensation, additional shares authorized (in shares) 5,539,886                
Subsequent Event | Employee Stock Purchase Plan (ESPP)                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based compensation, evergreen shares (in shares) 700,000