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INVESTMENTS (Schedule of cash, cash equivalents and short-term investments) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]          
Cash and cash equivalents $ 53,782 $ 63,422 $ 53,782 $ 63,422 $ 56,766
Short-term Investments          
Amortized cost 555,214   555,214   382,100
Unrealized Gains 1,637   1,637   120
Unrealized Losses (45)   (45)   (496)
Estimated fair value 556,806   556,806   381,724
Cash, cash equivalents and short-term investments, amortized cost 608,996   608,996   438,866
Cash, cash equivalents and short-term investments, estimated fair value 610,588   610,588   438,490
Interest income and gains on short-term investments, net 3,792 $ 972 6,795 $ 1,939  
Unrealized losses, greater than 12 months 0   0    
Cash and cash equivalents          
Debt Securities, Available-for-sale [Line Items]          
Cash and cash equivalents 49,734   49,734   55,501
Cash and cash equivalents, estimated fair value 49,734   49,734   55,501
Money market funds          
Debt Securities, Available-for-sale [Line Items]          
Cash and cash equivalents 4,048   4,048   1,265
Cash and cash equivalents, estimated fair value 4,048   4,048   1,265
Certificates of deposit          
Short-term Investments          
Amortized cost 149,074   149,074   95,080
Unrealized Gains 39   39   1
Unrealized Losses (5)   (5)   (43)
Estimated fair value 149,108   149,108   95,038
Government debt securities          
Short-term Investments          
Amortized cost 149,683   149,683   100,449
Unrealized Gains 474   474   92
Unrealized Losses (13)   (13)   (63)
Estimated fair value 150,144   150,144   100,478
Corporate debt securities          
Short-term Investments          
Amortized cost 256,457   256,457   186,571
Unrealized Gains 1,124   1,124   27
Unrealized Losses (27)   (27)   (390)
Estimated fair value 257,554   257,554   186,208
Cash and cash equivalents          
Debt Securities, Available-for-sale [Line Items]          
Cash and cash equivalents 53,782   53,782   56,766
Cash and cash equivalents, estimated fair value $ 53,782   $ 53,782   $ 56,766