D.INVESTMENTS

As of March 31, 2015 and December 31, 2014, our investments equaled $117.3 million and $24.9 million, respectively, and consisted of securities classified as available-for-sale.

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

	March 31, 2015
			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
	Cost		Gains		Losses			Value
Corporate debt securities
Due in one year or less	$	15,167	$	13	$	—		$	15,180
Due in one to three years	47,352		45		(10		)	47,387
Commercial paper
Due in one year or less	30,460		2		(1		)	30,461
Due in one to three years	—		—		—			—
Certificates of deposit
Due in one year or less	15,000		—		(1		)	14,999
Due in one to three years	—		—		—			—
Municipal securities
Due in one year or less	—		—		—			—
Due in one to three years	9,315		—		(4		)	9,311
Total investments	$	117,294	$	60	$	(16	)	$	117,338

	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

The $92.4 million increase in our total investments was primarily due to the sale of approximately 4.6 million shares of our common stock at a public offering price of $44.00 per share in March 2015, resulting in gross proceeds to us of approximately $201.2 million, prior to underwriting discounts of $12.1 million and $0.2 million in commissions and other offering expenses.

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 month periods ended March 31, 2015 and 2014. 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, 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 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.