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 June 30, 2017 and December 31, 2016. There was no transfer of any assets and liabilities between Levels during the three months ended June 30, 2017 or the year ended December 31, 2016.

The following table summarizes our investments by significant categories and fair value measurements on a recurring basis as of June 30, 2017 and December 31, 2016.



(in thousands)	Maturity (in years)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
June 30, 2017
Level 1:
Investments:
U.S. treasuries	< 3	$	39,900	$	5	$	(107	)	$	39,798
Mutual funds	N/A	500		7		—			507
Total investments		$	40,400	$	12	$	(107	)	$	40,305
Level 2:
Certificates of deposit	< 1	$	2,322	$	—	$	—		$	2,322
Total		$	42,722	$	12	$	(107	)	$	42,627

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		$	54,196	$	30	$	(77	)	$	54,149

There were no realized gains or losses for the six months ended June 30, 2017 and 2016. We had $0.1 million gross unrealized losses in our U.S. Treasury securities as of June 30, 2017 and December 31, 2016, respectively. The fair value of these securities in an unrealized loss position represented 87% and 58% of the total fair value of all U.S. Treasury securities as of June 30, 2017 and December 31, 2016, 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 and intent 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 June 30, 2017 and December 31, 2016 was $443.8 million and $462.9 million, respectively. The estimated fair values of our notes payable at June 30, 2017 and December 31, 2016 were $446.0 million and $462.9 million, respectively. These valuations are considered Level II in the hierarchy for fair value measurement and are based on quoted market prices.