MARKETABLE SECURITIES - Schedule of Available For Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale securities
Amortized Cost	$ 823,799	$ 175,082
Gains in Accumulated Other Comprehensive Income (Loss)	1,084	12
Losses in Accumulated Other Comprehensive Income (Loss)	0	(87)
Estimated Fair Value	824,883	175,007
Cash equivalents
Available-for-sale securities
Amortized Cost	349,975	30,320
Gains in Accumulated Other Comprehensive Income (Loss)	2	2
Losses in Accumulated Other Comprehensive Income (Loss)	0	0
Estimated Fair Value	349,977	30,322
Marketable securities
Available-for-sale securities
Amortized Cost	473,824	144,762
Gains in Accumulated Other Comprehensive Income (Loss)	1,082	10
Losses in Accumulated Other Comprehensive Income (Loss)	0	(87)
Estimated Fair Value	474,906	144,685
U.S. government agency securities \| Cash equivalents
Available-for-sale securities
Amortized Cost	349,975	30,320
Gains in Accumulated Other Comprehensive Income (Loss)	2	2
Losses in Accumulated Other Comprehensive Income (Loss)	0	0
Estimated Fair Value	349,977	30,322
U.S. government agency securities \| Marketable securities
Available-for-sale securities
Amortized Cost	257,102	140,745
Gains in Accumulated Other Comprehensive Income (Loss)	60	10
Losses in Accumulated Other Comprehensive Income (Loss)	0	(73)
Estimated Fair Value	257,162	140,682
Corporate bonds \| Marketable securities
Available-for-sale securities
Amortized Cost	191,651	4,017
Gains in Accumulated Other Comprehensive Income (Loss)	970	0
Losses in Accumulated Other Comprehensive Income (Loss)	0	(14)
Estimated Fair Value	192,621	$ 4,003
Certificates of deposit \| Marketable securities
Available-for-sale securities
Amortized Cost	10,000
Gains in Accumulated Other Comprehensive Income (Loss)	1
Losses in Accumulated Other Comprehensive Income (Loss)	0
Estimated Fair Value	10,001
Asset backed securities \| Marketable securities
Available-for-sale securities
Amortized Cost	15,071
Gains in Accumulated Other Comprehensive Income (Loss)	51
Losses in Accumulated Other Comprehensive Income (Loss)	0
Estimated Fair Value	$ 15,122

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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