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INVESTMENTS (Tables)

3 Months Ended

Mar. 31, 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
March 31, 2019		Cost			Gains			Losses			Value			Months			Months
Available-for-sale
Short-term:
Commercial paper		$	38,964		$	—		$	—		$	38,964		$	—		$	—
Certificates of deposit			5,024			—			—			5,024			—			—
Municipal securities			103,664			30			4			103,690			4			—
U.S. government agency securities			19,940			7			2			19,945			2			—
U.S. treasuries			89,267			13			12			89,268			12			—
Variable rate demand notes			6,806			—			—			6,806			—			—
Total		$	263,665		$	50		$	18		$	263,697		$	18		$	—

														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

		March 31, 2019						December 31, 2018
		Amortized Cost			Fair Value			Amortized Cost				Fair Value
Less than 1 year:
Commercial paper		$	38,964		$	38,964		$	52,838		$	52,838
Municipal securities			103,664			103,690			151,690			151,644
U.S. government agency securities			19,940			19,945			19,943			19,931
Certificates of deposit			5,024			5,024			14,075			14,075
U.S. treasuries			89,267			89,268			78,189			78,157
Due 21 - 30 years:
Variable rate demand notes			5,805			5,805			4,005			4,005
Due 31 - 40 years:
Variable rate demand notes			1,001			1,001			—			—
Total		$	263,665		$	263,697		$	320,740		$	320,650

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Tabular disclosure of unrealized gains and losses on investments.

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