4.INVESTMENTS

The following table summarizes the Company’s investments at:

March 31, 2018	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Fair Value		Continuous Unrealized Loss Position less than 12 Months		Continuous Unrealized Loss Position greater than 12 Months
Available-for-sale
Short-term:
Commercial paper	$	42,288	$	-	$	-	$	42,288	$	-	$	-
Municipal securities	441,360		-		508		440,852		508		-
U.S. government agency securities	74,773		-		107		74,666		107		-
Variable rate demand notes	27,416		-		-		27,416		-		-
Long-term:
Municipal securities	527		-		-		527		-		-
U.S. government agency securities	2,973		-		3		2,970		3		-

Total	$	589,337	$	-	$	618	$	588,719	$	618	$	-

December 31, 2017	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Fair Value		Continuous Unrealized Loss Position less than 12 Months		Continuous Unrealized Loss Position greater than 12 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 three-months ended March 31, 2018 and 2017, realized gains or losses recognized on the sale of investments were not significant.

The Company’s investments at March 31, 2018 and December 31, 2017 in commercial paper, certificates of deposit, municipal securities, U.S. government agency securities 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, 2018				December 31, 2017

	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than 1 year:
Commercial paper	$	42,288	$	42,288	$	81,026	$	81,026
Municipal securities	441,360		440,852		469,604		468,865
U.S. government agency securities	74,773		74,666		61,307		61,219
Certificates of deposit	-		-		11,869		11,869
Due 1 -10 years:
Municipal securities	527		527		-		-
U.S. government agency securities	2,973		2,970		2,369		2,366
Variable rate demand notes	6,622		6,622		6,366		6,366
Due 11 - 20 years:
Variable rate demand notes	18,542		18,542		28,377		28,377
Due 21 - 30 years:
Variable rate demand notes	2,252		2,252		15,211		15,211

Total	$	589,337	$	588,719	$	676,129	$	675,299