INVESTMENTS (Tables)

9 Months Ended

Sep. 30, 2020

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
September 30, 2020	Cost		Gains		Losses		Value		Months		Months
Available-for-sale
Short-term:
Commercial paper	$	91,445	$	—	$	—	$	91,445	$	—	$	—
Certificates of deposit		19,959		—		—		19,959		—		—
U.S. government agency securities		66,435		21		3		66,453		3		—
U.S. treasuries		421,093		398		22		421,469		22		—
Long-term:
U.S. government agency securities		10,205		—		1		10,204		1		—
U.S. treasuries		10,370		—		3		10,367		3		—
Total	$	619,507	$	419	$	29	$	619,897	$	29	$	—

									Continuous		Continuous
			Gross		Gross				Unrealized		Unrealized
			Unrealized		Unrealized				Loss Position		Loss Position
	Amortized		Holding		Holding		Fair		less than 12		greater than 12
December 31, 2019	Cost		Gains		Losses		Value		Months		Months
Available-for-sale
Short-term:
Commercial paper	$	83,478	$	—	$	—	$	83,478	$	—	$	—
Certificates of deposit		28,049		—		—		28,049		—		—
Municipal securities		147,983		145		20		148,108		20		—
U.S. government agency securities		40,620		5		35		40,590		35		—
U.S. treasuries		211,055		134		31		211,158		31		—
Variable rate demand notes		21,680		—		—		21,680		—		—
Long-term:
Municipal securities		1,562		—		1		1,561		1		—
U.S. government agency securities		5,267		—		1		5,266		1		—
U.S. treasuries		6,077		1		—		6,078		—		—
Total	$	545,771	$	285	$	88	$	545,968	$	88	$	—

Summary of the underlying contractual maturities of investments

	September 30, 2020				December 31, 2019
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than 1 year:
Commercial paper	$	91,445	$	91,445	$	83,478	$	83,478
Municipal securities		—		—		147,983		148,108
U.S. government agency securities		66,435		66,453		40,620		40,590
Certificates of deposit		19,959		19,959		28,049		28,049
U.S. treasuries		421,093		421,469		211,055		211,158
Due 1 -10 years:
Municipal securities		—		—		1,562		1,561
U.S. treasuries		10,370		10,367		6,077		6,078
U.S. government agency securities		10,205		10,204		5,267		5,266
Variable rate demand notes		—		—		3,905		3,905
Due 11 - 20 years:
Variable rate demand notes		—		—		8,886		8,886
Due 21 - 30 years:
Variable rate demand notes		—		—		6,885		6,885
Due 31 - 40 years:
Variable rate demand notes		—		—		2,004		2,004
Total	$	619,507	$	619,897	$	545,771	$	545,968