MARKETABLE SECURITIES - Schedule of Underlying Maturities of AFS Securities (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Available-for-sale securities
Due in one year or less, Cost	$ 758,821
Due in one year or less, Fair Value	759,336
Due after one year through four years, Cost	64,978
Due after one year through four years, Fair Value	65,547
Cash equivalents
Available-for-sale securities
Due in one year or less, Cost	349,975
Due in one year or less, Fair Value	349,977
Cash equivalents | U.S. government agency securities
Available-for-sale securities
Due in one year or less, Cost	349,975
Due in one year or less, Fair Value	349,977
Marketable securities
Available-for-sale securities
Due in one year or less, Cost	408,846
Due in one year or less, Fair Value	409,359
Due after one year through four years, Cost	64,978
Due after one year through four years, Fair Value	65,547
Marketable securities | U.S. government agency securities
Available-for-sale securities
Due in one year or less, Cost	249,943
Due in one year or less, Fair Value	249,949
Due after one year through four years, Cost	7,159
Due after one year through four years, Fair Value	7,213
Marketable securities | Corporate bonds
Available-for-sale securities
Due in one year or less, Cost	148,903
Due in one year or less, Fair Value	149,409
Due after one year through four years, Cost	42,748
Due after one year through four years, Fair Value	43,212
Marketable securities | Certificates of deposit
Available-for-sale securities
Due in one year or less, Cost	10,000
Due in one year or less, Fair Value	10,001
Due after one year through four years, Cost	0
Due after one year through four years, Fair Value	0
Marketable securities | Asset backed securities
Available-for-sale securities
Due in one year or less, Cost	0
Due in one year or less, Fair Value	0
Due after one year through four years, Cost	15,071
Due after one year through four years, Fair Value	$ 15,122