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INVESTMENTS (Tables)

9 Months Ended

Sep. 30, 2019

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, 2019		Cost			Gains			Losses			Value			Months			Months
Available-for-sale
Short-term:
Commercial paper		$	81,869		$	—		$	—		$	81,869		$	—		$	—
Certificates of deposit			2,502			—			—			2,502			—			—
Municipal securities			138,072			151			9			138,214			9			—
U.S. government agency securities			41,186			7			37			41,156			37			—
U.S. treasuries			294,099			179			10			294,268			10			—
Variable rate demand notes			29,347			—			—			29,347			—			—
Long-term:
U.S. treasuries			11,174			7			4			11,177			4			—
Municipal securities			3,198			—			5			3,193			5			—
Total		$	601,447		$	344		$	65		$	601,726		$	65		$	—


														Continuous			Continuous
					Gross			Gross						Unrealized			Unrealized
					Unrealized			Unrealized						Loss Position			Loss Position
		Amortized			Holding			Holding			Fair			less than 12			greater than 12
December 31, 2018		Cost			Gains			Losses			Value			Months			Months
Available-for-sale
Short-term:
Commercial paper		$	52,838		$	—		$	—		$	52,838		$	—		$	—
Certificates of deposit			14,075			—			—			14,075			—			—
Municipal securities			151,690			16			62			151,644			62			—
U.S. government agency securities			19,943			—			12			19,931			12			—
U.S. treasuries			78,189			—			32			78,157			32			—
Variable rate demand notes			4,005			—			—			4,005			—			—
Total		$	320,740		$	16		$	106		$	320,650		$	106		$	—

Summary of the underlying contractual maturities of investments


		September 30, 2019						December 31, 2018
		Amortized Cost			Fair Value			Amortized Cost			Fair Value
Less than 1 year:
Commercial paper		$	81,869		$	81,869		$	52,838		$	52,838
Municipal securities			138,072			138,214			151,690			151,644
U.S. government agency securities			41,186			41,156			19,943			19,931
Certificates of deposit			2,502			2,502			14,075			14,075
U.S. treasuries			294,099			294,268			78,189			78,157
Due 1 - 10 years:
Municipal securities			3,198			3,193			—			—
U.S. treasuries			11,174			11,177			—			—
Variable rate demand notes			5,412			5,412			—			—
Due 11 - 20 years:
Variable rate demand notes			12,197			12,197			—			—
Due 21 - 30 years:
Variable rate demand notes			9,721			9,721			4,005			4,005
Due 31 - 40 years:
Variable rate demand notes			2,017			2,017			—			—
Total		$	601,447		$	601,726		$	320,740		$	320,650