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SUBSEQUENT EVENTS - tZero Issuance of Equity Awards (Details) - USD ($)
1 Months Ended 12 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 23, 2018
Jan. 22, 2018
Subsequent Event [Line Items]              
Stock-based compensation expense     $ 4,077,000 $ 4,891,000 $ 3,526,000    
Stock options              
Subsequent Event [Line Items]              
tZero restricted stock units authorized, percentage   5.00%          
Stock-based compensation expense     $ 0 $ 0 $ 0    
Restricted Stock Units | Subsequent Event              
Subsequent Event [Line Items]              
tZero restricted stock units granted, percentage 1.00%            
Stock-based compensation expense $ 4,000,000            
tZero.com, Inc. | Subsequent Event              
Subsequent Event [Line Items]              
Ownership percentage           80.00% 81.00%