D.INVESTMENTS

As of September 30, 2015 and December 31, 2014, our investments equaled $275.3 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 September 30, 2015 and December 31, 2014 (in thousands):


	September 30, 2015
			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Corporate debt securities
Due in one year or less	$	26,244	$	8	$	(10)	$	26,242
Due in one to three years		147,615		25		(276)		147,364
Commercial paper
Due in one year or less		7,488		2		—		7,490
Certificates of deposit
Due in one year or less		10,000		1		—		10,001
Municipal securities
Due in one year or less		13,874		11		—		13,885
Due in one to three years		70,249		117		(5)		70,361
Total investments	$	275,470	$	164	$	(291)	$	275,343


	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 $250.5 million increase in our total investments was primarily due to the $188.8 million of net proceeds from our March 2015 public equity offering, as discussed in Note N, “Stockholders’ Equity.”

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 any of the three or nine month periods ended September 30, 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. 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.