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STOCK-BASED COMPENSATION - Summary of RSU Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Nov. 12, 2019
Feb. 28, 2018
Dec. 31, 2018
Sep. 30, 2019
Dec. 31, 2018
Weighted-Average Grant Date Fair Value          
Award vesting period       4 years  
Restricted securities units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock option expense       $ 21,519 $ 4,536
Restricted stock units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Compensation cost not yet recognized       3 years 21 days 2 years 5 months 1 day
Shares (in thousands)          
Unvested — Beginning balance (in shares)       381,000  
Granted (in shares)       1,792,000  
Vested (in shares)       (324,000)  
Forfeited (in shares)       (112,000)  
Unvested — Ending balance (in shares)     381,000 1,737,000 381,000
Weighted-Average Grant Date Fair Value          
Unvested — Beginning balance (in usd per share)       $ 18.11  
Granted (in usd per share)       15.86  
Vested (in usd per share)       14.65  
Forfeited (in usd per share)       19.02  
Unvested — Ending balance (in usd per share)     $ 18.11 $ 16.38 $ 18.11
Performance Shares [Member]          
Shares (in thousands)          
Vested (in shares)     (850,000)    
Forfeited (in shares)         (25,000)
Unvested — Ending balance (in shares)   875,000   1,252,500  
Subsequent Event | Restricted stock units          
Shares (in thousands)          
Granted (in shares) 186,309        
Weighted-Average Grant Date Fair Value          
Award vesting period 4 years        
Compensation not yet recognized, awards other than options $ 6,800        
Maximum | Employees and non-employee directors | Restricted stock units          
Weighted-Average Grant Date Fair Value          
Award vesting period       4 years