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DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NARRATIVE (Details)
12 Months Ended
Dec. 31, 2020
USD ($)
period
segment
$ / shares
Dec. 31, 2019
USD ($)
Concentration Risk [Line Items]    
Minimum shares allowed to purchase under fractional shares program 0.000001  
Number of operating segments | segment 1  
Term of user contract 30 days  
Advertising costs | $ $ 157,100,000 $ 119,600,000
Research and development costs | $ $ 52,200,000 $ 27,700,000
Dividend yield 0.00%  
Settlement period for receivables from brokers, dealers, and clearing organizations 30 days  
Percentage of users to receive stock reward 98.00%  
Maximum annual reward earned by user | $ $ 500  
Claim period of reward 60 days  
Realized and unrealized gains and losses | $ $ 3,000,000  
Contractual termination period for securities borrowed and loaned 3 days  
Performance-based RSUs    
Concentration Risk [Line Items]    
Vesting period 4 years  
Market-Based RSUs    
Concentration Risk [Line Items]    
Vesting period 6 years  
Number of service periods | period 2  
Minimum    
Concentration Risk [Line Items]    
Potential value of stock reward (in dollars per share) | $ / shares $ 2.50  
Actual value of stock reward (in dollars per share) | $ / shares 2.50  
Maximum    
Concentration Risk [Line Items]    
Potential value of stock reward (in dollars per share) | $ / shares 225  
Actual value of stock reward (in dollars per share) | $ / shares $ 10