5.INVESTMENTS

The following table summarizes the Company’s investments at:


									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	$	—

During the three-months ended March 31, 2019 and 2018, realized gains or losses recognized on the sale of investments were not significant.

The Company’s investments at March 31, 2019 and December 31, 2018 in commercial paper, certificates of deposit, municipal securities, U.S. government agency securities, U.S. treasuries and/or variable rate demand notes (“VRDNs”) carried investment grade credit ratings. VRDNs are floating rate municipal bonds with embedded put options that allow the bondholder to sell the security at par plus accrued interest. All of the put options are secured by a pledged liquidity source. While they are classified as marketable investment securities, the put option allows the VRDNs to be liquidated at par on a same day, or more generally, on a seven-day settlement basis.

The following table summarizes the underlying contractual maturities of the Company’s investments at:


	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