C. Investments

As of March 31, 2014 and December 31, 2013, our investments equaled $184.6 million and $186.8 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, 2014 and December 31, 2013 (in thousands):

	March 31, 2014
			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
	Cost		Gains		Losses			Value
Corporate debt securities
Due in one year or less	$	23,492	$	46	$	(2	)	$	23,536
Due in one to three years	108,536		159		(47		)	108,648
U.S. treasury and government agency securities
Due in one year or less	21,079		35		—			21,114
Due in one to three years	29,772		17		(28		)	29,761
Commercial paper
Due in one year or less	1,499		—		(1		)	1,498
Total investments	$	184,378	$	257	$	(78	)	$	184,557

	December 31, 2013
			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
	Cost		Gains		Losses			Value
Corporate debt securities
Due in one year or less	$	42,609	$	44	$	(4	)	$	42,649
Due in one to three years	91,443		137		(106		)	91,474
U.S. treasury and government agency securities
Due in one year or less	18,526		19		—			18,545
Due in one to three years	34,123		37		(25		)	34,135
Total investments	$	186,701	$	237	$	(135	)	$	186,803

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, 2014 and 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 March 31, 2014, an insignificant portion 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 which 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.

Realized Gains and Losses on Investments

Gains and losses are determined on the specific identification method. Realized gains were insignificant during the three months ended March 31, 2014 and 2013.