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AVAILABLE-FOR-SALE SECURITIES AND FAIR VALUE MEASUREMENTS - Available-for-Sale Securities (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Available-for-Sale Securities    
Amortized Cost $ 306,706 $ 108,977
Gross Unrealized Gains 35 0
Gross Unrealized Losses 0 (3)
Estimated Fair Value $ 306,741 108,974
Maturity period for marketable securities    
Maximum contractual maturity 1 year  
Weighted average maturity 3 months  
U.S. government securities    
Available-for-Sale Securities    
Amortized Cost $ 53,799 29,736
Gross Unrealized Gains 35 0
Gross Unrealized Losses 0 (3)
Estimated Fair Value 53,834 29,733
U.S. government agencies    
Available-for-Sale Securities    
Amortized Cost   4,971
Gross Unrealized Gains   0
Gross Unrealized Losses   0
Estimated Fair Value   4,971
U.S. corporate notes    
Available-for-Sale Securities    
Amortized Cost   2,875
Gross Unrealized Gains   0
Gross Unrealized Losses   0
Estimated Fair Value   2,875
U.S. commercial paper    
Available-for-Sale Securities    
Amortized Cost 18,915 22,037
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 18,915 22,037
Money market funds    
Available-for-Sale Securities    
Amortized Cost 233,992 49,358
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 233,992 $ 49,358