C. Investments

As of March 31, 2013 and December 31, 2012, our investments equaled $183.2 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 March 31, 2013 and December 31, 2012 (in thousands):

	March 31, 2013
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Corporate debt securities
Due in one year or less	$	56,956	$	105	$	(7	)	$	57,054
Due in one to three years	67,624		103		(71		)	67,656
U.S. treasury and government agency securities
Due in one year or less	22,635		54		—			22,689
Due in one to three years	31,757		72		(1		)	31,828
Commercial paper
Due in one year or less	3,998		—		(1		)	3,997
Total investments	$	182,970	$	334	$	(80	)	$	183,224

	December 31, 2012
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair 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

The following is a summary of the fair value of our investments with unrealized losses that are deemed to be temporarily impaired and their respective gross unrealized losses aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position as of March 31, 2013 and December 31, 2012 (in thousands):

	March 31, 2013
	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Corporate debt securities	$	53,936	$	(78	)	$	—	$	—	$	53,936	$	(78	)
U.S. treasury and government agency securities	4,542		(1		)	—		—		4,542		(1		)
Commercial paper	3,998		(1		)	—		—		3,998		(1		)
	$	62,476	$	(80	)	$	—	$	—	$	62,476	$	(80	)

	December 31, 2012
	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Corporate debt securities	$	37,036	$	(43	)	$	—	$	—	$	37,036	$	(43	)
U.S. treasury and government agency securities	6,271		(2		)	—		—		6,271		(2		)
Commercial paper	3,992		(4		)	—		—		3,992		(4		)
	$	47,299	$	(49	)	$	—	$	—	$	47,299	$	(49	)

We did not recognize any unrealized other-than-temporary impairment losses in our condensed consolidated statements of operations related to our securities during either of the three months ended March 31, 2013 or 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 months ended March 31, 2013 and 2012.