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INVESTMENTS (Schedule of Cash, Cash Equivalents and Short-Term Investments) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]          
Cash and cash equivalents, Amortized Cost $ 52,638 $ 64,259 $ 52,638 $ 64,259 $ 56,766
Short-term Investments, Amortized Cost 688,070   688,070   382,100
Cash, cash equivalents and short-term investments, Amortized Cost 740,708   740,708   438,866
Short-term Investments, Estimated Fair Value 689,823   689,823   381,724
Cash, cash equivalents and short-term investments, Estimated Fair Value 742,461   742,461   438,490
Interest income and gains on short-term investments, net 4,227 $ 1,459 11,021 $ 3,398  
Cash and cash equivalents          
Debt Securities, Available-for-sale [Line Items]          
Cash and cash equivalents, Amortized Cost 51,200   51,200   55,501
Cash and cash equivalents, Estimated Fair Value 51,200   51,200   55,501
Money market funds          
Debt Securities, Available-for-sale [Line Items]          
Cash and cash equivalents, Amortized Cost 1,438   1,438   1,265
Cash and cash equivalents, Estimated Fair Value 1,438   1,438   1,265
Certificates of deposit          
Debt Securities, Available-for-sale [Line Items]          
Short-term Investments, Amortized Cost 180,223   180,223   95,080
Short-term Investments, Estimated Fair Value 180,321   180,321   95,038
Government debt securities          
Debt Securities, Available-for-sale [Line Items]          
Short-term Investments, Amortized Cost 211,584   211,584   100,449
Short-term Investments, Estimated Fair Value 211,995   211,995   100,478
Corporate debt securities          
Debt Securities, Available-for-sale [Line Items]          
Short-term Investments, Amortized Cost 296,263   296,263   186,571
Short-term Investments, Estimated Fair Value 297,507   297,507   186,208
Cash and cash equivalents          
Debt Securities, Available-for-sale [Line Items]          
Cash and cash equivalents, Amortized Cost 52,638   52,638   56,766
Cash and cash equivalents, Estimated Fair Value $ 52,638   $ 52,638   $ 56,766