INVESTMENTS (Tables)

6 Months Ended

Jun. 30, 2021

INVESTMENTS

Summary of investments in available-for-sale

									Continuous		Continuous
			Gross		Gross				Unrealized		Unrealized
			Unrealized		Unrealized				Loss Position		Loss Position
	Amortized		Holding		Holding		Fair		less than 12		greater than 12
June 30, 2021	Cost		Gains		Losses		Value		Months		Months
Available-for-sale
Short-term:
Commercial paper	$	186,912	$	—	$	—	$	186,912	$	—	$	—
Certificates of deposit		43,240		2		—		43,242		—		—
U.S. government agency securities		64,914		10		2		64,922		2		—
U.S. treasuries		673,912		35		71		673,876		71		—
Long-term:
U.S. government agency securities		29,659		—		8		29,651		8		—
U.S. treasuries		61,423		—		41		61,382		41		—
Total	$	1,060,060	$	47	$	122	$	1,059,985	$	122	$	—

									Continuous		Continuous
			Gross		Gross				Unrealized		Unrealized
			Unrealized		Unrealized				Loss Position		Loss Position
	Amortized		Holding		Holding		Fair		less than 12		greater than 12
December 31, 2020	Cost		Gains		Losses		Value		Months		Months
Available-for-sale
Short-term:
Commercial paper	$	119,886	$	—	$	—	$	119,886	$	—	$	—
Certificates of deposit		20,387		—		—		20,387		—		—
Municipal securities		9,083		—		—		9,083		—		—
U.S. government agency securities		81,521		13		3		81,531		3		—
U.S. treasuries		650,386		150		69		650,467		69		—
Long-term:
U.S. government agency securities		10,350		1		—		10,351		—		—
U.S. treasuries		33,946		1		7		33,940		7		—
Total	$	925,559	$	165	$	79	$	925,645	$	79	$	—

Summary of the underlying contractual maturities of investments

	June 30, 2021				December 31, 2020
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than 1 year:
Commercial paper	$	186,912	$	186,912	$	119,886	$	119,886
Municipal securities		—		—		9,083		9,083
U.S. government agency securities		64,914		64,922		81,521		81,531
Certificates of deposit		43,240		43,242		20,387		20,387
U.S. treasuries		673,912		673,876		650,386		650,467
Due 1 - 10 years:
U.S. treasuries		61,423		61,382		33,946		33,940
U.S. government agency securities		29,659		29,651		10,350		10,351
Total	$	1,060,060	$	1,059,985	$	925,559	$	925,645