D. INVESTMENTS

As of March 31, 2016 and December 31, 2015, our investments equaled $276.8 million and $237.6 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 March 31, 2016 and December 31, 2015 (in thousands):


	March 31, 2016
			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Corporate debt securities
Due in one year or less	$	56,886	$	7	$	(43)	$	56,850
Due in one to three years		193,452		596		(89)		193,959
U.S. treasury and government agency securities
Due in one year or less		4,496		32		—		4,528
Commercial paper
Due in one year or less		18,973		4		—		18,977
Municipal securities
Due in one year or less		2,500		2		—		2,502
Total investments	$	276,307	$	641	$	(132)	$	276,816


	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

Impairments and Unrealized Gains and Losses on Investments

We did not recognize any other-than-temporary impairment losses in our condensed consolidated statements of operations related to our securities during the three months ended March 31, 2016 and 2015. 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 March 31, 2016, 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.