C. Investments

As of June 30, 2013 and December 31, 2012, our investments equaled $184.6 million and $180.8 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 June 30, 2013 and December 31, 2012 (in thousands):

	June 30, 2013
			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
	Cost		Gains		Losses			Value
Corporate debt securities
Due in one year or less	$	54,636	$	76	$	(12	)	$	54,700
Due in one to three years	73,115		35		(431		)	72,719
U.S. treasury and government agency securities
Due in one year or less	24,683		32		(2		)	24,713
Due in one to three years	30,018		42		(82		)	29,978
Commercial paper
Due in one year or less	2,500		1		—			2,501
Total investments	$	184,952	$	186	$	(527	)	$	184,611

	December 31, 2012
			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
	Cost		Gains		Losses			Value
Corporate debt securities
Due in one year or less	$	52,332	$	88	$	(6	)	$	52,414
Due in one to three years	59,176		137		(37		)	59,276
U.S. treasury and government agency securities
Due in one year or less	24,795		86		—			24,881
Due in one to three years	34,606		84		(2		)	34,688
Commercial paper
Due in one year or less	9,494		1		(4		)	9,491
Total investments	$	180,403	$	396	$	(49	)	$	180,750

Impairments and Unrealized Losses on Investments

We did not recognize any unrealized other-than-temporary impairment losses in our condensed consolidated statements of operations related to our securities during any of the three or six months ended June 30, 2013 and 2012. 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 both the three and six months ended June 30, 2013. During both the three and six months ended June 30, 2012 we recorded realized losses of $1.5 million to our condensed consolidated statements of operations related to the sale of our then-remaining auction rate securities portfolio.