3. 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
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	$	—


									Continuous		Continuous
			Gross		Gross				Unrealized		Unrealized
			Unrealized		Unrealized				Loss Position		Loss Position
	Amortized		Holding		Holding		Fair		less than 12		greater than 12
December 31, 2017	Cost		Gains		Losses		Value		Months		Months
Available-for-sale
Short-term:
Commercial paper	$	81,026	$	—	$	—	$	81,026	$	—	$	—
Certificates of deposit		11,869		—		—		11,869		—		—
Municipal securities		469,604		1		740		468,865		740		—
U.S. government agency securities		61,307		—		88		61,219		88		—
Variable rate demand notes		49,954		—		—		49,954		—		—
Long-term:
U.S. government agency securities		2,369		—		3		2,366		3		—
Total	$	676,129	$	1	$	831	$	675,299	$	831	$	—

During the years ended December 31, 2018 and 2017, realized gains or losses recognized on the sale of investments were not significant.

The Company’s investments at December 31, 2018 and 2017 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:


	December 31, 2018				December 31, 2017
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than 1 year:
Commercial paper	$	52,838	$	52,838	$	81,026	$	81,026
Municipal securities		151,690		151,644		469,604		468,865
U.S. government agency securities		19,943		19,931		61,307		61,219
Certificates of deposit		14,075		14,075		11,869		11,869
U.S. treasuries		78,189		78,157		—		—
Due 1 - 10 years:
U.S. government agency securities		—		—		2,369		2,366
Variable rate demand notes		—		—		6,366		6,366
Due 11 - 20 years:
Variable rate demand notes		—		—		28,377		28,377
Due 21 - 30 years:
Variable rate demand notes		4,005		4,005		15,211		15,211
Total	$	320,740	$	320,650	$	676,129	$	675,299