FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-Sale Securities
Amortized Cost	$ 204,808	$ 306,706
Gross Unrealized Gains	0	35
Gross Unrealized Losses	0	0
Estimated Fair Value	204,808	306,741
Credit loss	0
U.S. government securities
Available-for-Sale Securities
Amortized Cost		53,799
Gross Unrealized Gains		35
Gross Unrealized Losses		0
Estimated Fair Value		53,834
U.S. commercial paper
Available-for-Sale Securities
Amortized Cost		18,915
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		18,915
Money market funds
Available-for-Sale Securities
Amortized Cost	204,808	233,992
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 204,808	$ 233,992

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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Amount of allowance for credit loss 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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