D. INVESTMENTS

As of December 31, 2015 and 2014, our investments equaled $237.6 million and $24.9 million, respectively, and consisted of securities classified as available-for-sale in accordance with accounting standards which provide guidance related to accounting and classification of certain investments in debt and equity securities.

The following is a summary of our investments as of December 31, 2015 and 2014 (in thousands):


	December 31, 2015
			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Corporate debt securities
Due in one year or less	$	27,964	$	—	$	(38)	$	27,926
Due in one to three years		173,652		3		(904)		172,751
Commercial paper
Due in one year or less		34,452		2		(5)		34,449
Municipal securities
Due in one year or less		2,500		—		—		2,500
Total investments	$	238,568	$	5	$	(947)	$	237,626


	December 31, 2014
			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Corporate debt securities
Due in one year or less	$	11,656	$	3	$	(4)	$	11,655
Due in one to three years		13,258		10		(33)		13,235
Total investments	$	24,914	$	13	$	(37)	$	24,890

Impairments and Unrealized Gains and Losses on Investments

We did not recognize any other-than-temporary impairment losses in our consolidated statements of operations related to our securities during 2015, 2014 or 2013. We considered various factors, including the length of time that each security was in an unrealized loss position and our ability and intent to hold these securities until the recovery of their amortized cost basis occurs. As of December 31, 2015, none of our investments has been in an unrealized loss position for more than one year. Future events may occur, or additional information may become available, which may cause us to identify credit losses where we do not expect to receive cash flows sufficient to recover the entire amortized cost basis of a security and may necessitate the recording of future realized losses on securities in our portfolio. Significant losses in the estimated fair values of our investments could have a material adverse effect on our earnings in future periods.