FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS

Cash, Cash equivalents and Restricted Investments

We classify our cash, cash equivalents and restricted investments in marketable securities within Level I in the fair value hierarchy because we use quoted market prices to determine the fair value. We classify our certificates of deposit within Level II in the fair value hierarchy as we use a market approach that compares fair values on certificates with similar maturities. We have no available for sale securities included in Level III as of December 31, 2016 and 2015. There was no transfer of any assets and liabilities between Levels during years ended December 31, 2016 and 2015.

The following table summarizes our investments by significant categories and fair value measurement on a recurring basis as of December 31, 2016 and 2015:



(In thousands)	Maturity (in years)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
December 31, 2016
Level 1:
Investments:
U.S. treasuries	< 3	$	51,376	$	25	$	(77	)	$	51,324
Mutual funds	N/A	500		5		—			505
Total investments		$	51,876	$	30	$	(77	)	$	51,829
Level 2:
Certificates of deposit	< 1	$	2,320	$	—	$	—		2,320
Total									$	54,149

December 31, 2015
Level 1:
Investments
U.S. treasuries	< 4	$	64,226	$	9	$	(144	)	$	64,091
Mutual funds	N/A	500		4		—			504
Total investments		$	64,726	$	13	$	(144	)	$	64,595
Level 2:
Certificates of deposit	< 1	$	2,319	$	—	$	—		$	2,319
Total									$	66,914

There were de minimis realized gains or losses for the years ended December 31, 2016 and 2015. We had $0.1 million gross unrealized losses in our U.S. Treasury securities as of December 31, 2016 and 2015. The fair value of these securities in an unrealized loss position represented 58% and 81% of the total fair value of all U.S. Treasury securities as of December 31, 2016 and 2015, respectively.

Unrealized losses are principally caused by changes in interest rates. In analyzing an issuer's financial condition, we consider whether the securities are issued by the federal government or its agencies, whether downgrades by bond rating agencies have occurred, and industry analysts' reports. As we have the ability to hold these available for sale marketable securities until maturity, or for the foreseeable future, no decline was deemed to be other-than-temporary.

Notes Payable

The carrying value of our notes payable at December 31, 2016 and 2015 was $462.9 million and $499.6 million, respectively, which approximated fair value. These valuations are considered Level II in the hierarchy for fair value measurement. At December 31, 2016, the valuation was based on readily available quoted market prices. The discounted cash flow method of valuation was used as of December 31, 2015.