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INVESTMENTS IN DEBT SECURITIES - Short-Term and Long-Term Investments (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 2,320,232  
Fair Value 2,319,916  
Short-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 868,162 $ 692,901
Gross Unrealized Gains 875 2,478
Gross Unrealized Losses (228) (267)
Fair Value 868,809 695,112
Long-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 1,452,070 461,455
Gross Unrealized Gains 499 2,552
Gross Unrealized Losses (1,462) (57)
Fair Value 1,451,107 463,950
U.S. agency securities | Short-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 93,867 153,386
Gross Unrealized Gains 227 782
Gross Unrealized Losses (6) (164)
Fair Value 94,088 154,004
U.S. agency securities | Long-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 152,820 168,762
Gross Unrealized Gains 46 519
Gross Unrealized Losses (175) (3)
Fair Value 152,691 169,278
Corporate bonds | Short-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 290,231 76,957
Gross Unrealized Gains 369 256
Gross Unrealized Losses (77) (14)
Fair Value 290,523 77,199
Corporate bonds | Long-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 704,676 174,655
Gross Unrealized Gains 337 1,401
Gross Unrealized Losses (916) (42)
Fair Value 704,097 176,014
Commercial paper | Short-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 141,551 4,999
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 141,551 4,999
Municipal securities | Short-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 15,738 10,377
Gross Unrealized Gains 12 57
Gross Unrealized Losses 0 (3)
Fair Value 15,750 10,431
Municipal securities | Long-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 21,952 1,045
Gross Unrealized Gains 23 15
Gross Unrealized Losses (10) 0
Fair Value 21,965 1,060
Certificates of deposit | Short-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 8,200  
Gross Unrealized Gains 0  
Gross Unrealized Losses 0  
Fair Value 8,200  
U.S. government securities | Short-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 308,080 404,194
Gross Unrealized Gains 248 1,244
Gross Unrealized Losses (5) (4)
Fair Value 308,323 405,434
U.S. government securities | Long-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 543,483 91,642
Gross Unrealized Gains 34 433
Gross Unrealized Losses (337) (2)
Fair Value 543,180 92,073
Foreign government securities | Short-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 10,495 42,988
Gross Unrealized Gains 19 139
Gross Unrealized Losses (140) (82)
Fair Value 10,374 43,045
Foreign government securities | Long-term debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 29,139 25,351
Gross Unrealized Gains 59 184
Gross Unrealized Losses (24) (10)
Fair Value $ 29,174 $ 25,525