MARKETABLE SECURITIES AND FAIR VALUE MEASUREMENTS

The Company’s noncurrent restricted cash and investments include $62.7 million of available-for-sale marketable securities and $40.8 million of cash collateral at March 31, 2016. The Company’s restricted investments within WSE-related assets include $2.3 million of certificates of deposit as of March 31, 2016. The available-for-sale marketable securities as of March 31, 2016 and December 31, 2015 consist of the following (in thousands):



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
March 31, 2016:
U.S. treasuries	$	62,706	$	185	$	—		$	62,891
Mutual funds	500		8		—			508
Total investments	$	63,206	$	193	$	—		$	63,399
December 31, 2015:
U.S. treasuries	$	64,226	$	9	$	(144	)	$	64,091
Mutual funds	500		4		—			504
Total investments	$	64,726	$	13	$	(144	)	$	64,595

There were no realized gains or losses for the three months ended March 31, 2016 and 2015. As of March 31, 2016 and December 31, 2015, the contractual maturities of the U.S. treasuries were one to four years.

As of March 31, 2016, none of the Company’s U.S. treasuries were in an unrealized loss position. Unrealized losses are principally due to changes in interest rates. In analyzing an issuer’s financial condition, the Company considers whether the securities are issued by the federal government or its agencies, whether downgrades by bond rating agencies have occurred, and industry analysts’ reports. The fair value of these securities in an unrealized loss position represented 0% and 81% of the total fair value of all securities available for sale as of March 31, 2016 and December 31, 2015, respectively. As the Company has the ability and intent to hold debt securities until maturity, or for the foreseeable future as classified as available for sale, no decline was deemed to be other-than-temporary.

Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a liability.

As a basis for considering such assumptions, the Company uses a three-tier value hierarchy, which prioritizes the inputs used in measuring fair value as follows:


•	Level I—observable inputs such as quoted prices in active markets


•	Level II—inputs other than the quoted prices in active markets that are observable either directly or indirectly


•	Level III—unobservable inputs in which there is little or no market data, which requires the Company to develop its own assumptions

This hierarchy requires the Company to use observable market data when available and to minimize the use of unobservable inputs when determining fair value.

The following table summarizes the Company’s financial assets measured at fair value on a recurring basis (in thousands):



	Total Fair Value		Level I		Level II		Level III
March 31, 2016:
Certificates of deposit	$	2,319	$	2,319	$	—	$	—
U.S. treasuries	62,891		62,891		—		—
Mutual funds	508		508		—		—
Total	$	65,718	$	65,718	$	—	$	—
December 31, 2015:
Certificates of deposit	$	2,319	$	2,319	$	—	$	—
U.S. treasuries	64,091		64,091		—		—
Mutual funds	504		504		—		—
Total	$	66,914	$	66,914	$	—	$	—

There were no transfers between Level I and Level II assets during the three months ended March 31, 2016 or the year ended December 31, 2015.

As of March 31, 2016 and December 31, 2015, certificates of deposit consisted of certificates of deposit held by domestic financial institutions, which are presented as restricted investments within WSE-related assets in the accompanying consolidated balance sheets.

The carrying value of the Company’s financial instruments not measured at fair value, including cash and restricted cash, approximates fair value due to the relatively short maturity of such instruments. The fair value of these instruments would be categorized as Level II of the fair value hierarchy, with the exception of cash and cash equivalents, which would be categorized as Level I.

At March 31, 2016 and December 31, 2015, the carrying value of the Company’s notes payable of $494.5 million and $499.6 million, respectively, approximated fair value. The estimated fair values of the Company’s notes payable are considered a Level II valuation in the hierarchy for fair value measurement and are based on a cash flow model discounted at market interest rates that considers the underlying risks of unsecured debt.