Short-term Investments

3 Months Ended

Mar. 31, 2015

Investments, Debt and Equity Securities [Abstract]

Short-term Investments

5. Short-term Investments

Short-term investments consisted of available-for-sale securities as follows (in thousands):

Type of security as of March 31, 2015	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Corporate debt securities	$	38,759	$	1	$	(18	)	$	38,742
Asset-backed securities		2,419		—		(1	)		2,418
Total available-for-sale securities	$	41,178	$	1	$	(19	)	$	41,160

Type of security as of December 31, 2014	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
U.S. government-related debt securities	$	4,502	$	—	$	(4	)	$	4,498
Corporate debt securities		47,345		2		(40	)		47,307
Asset-backed securities		3,198		—		(1	)		3,197
Total available-for-sale securities	$	55,045	$	2	$	(45	)	$	55,002

The fair values of available-for-sale securities by contractual maturity were as follows (in thousands):

Contractual maturity	March 31, 2015		December 31, 2014
Maturing in one year or less	$	38,612	$	51,235
Maturing in one to three years		2,548		3,767
Total available-for-sale securities	$	41,160	$	55,002

The following table summarizes investments that have been in a continuous unrealized loss position for less than 12 months and a continuous unrealized loss position for more than 12 months as of March 31, 2015 (in thousands):

	Less Than 12 Months					12 Months or Greater					Total
	Fair value		Gross unrealized losses			Fair value		Gross unrealized losses			Fair value		Gross unrealized losses
Corporate debt securities	$	28,340	$	(17	)	$	1,999	$	(1	)	$	30,339	$	(18	)
Asset-backed securities		2,418		(1	)		—		—			2,418		(1	)
Total	$	30,758	$	(18	)	$	1,999	$	(1	)	$	32,757	$	(19	)

The Company invests in securities that are rated investment grade or better. The unrealized losses on investments as of March 31, 2015 and December 31, 2014 were primarily caused by interest rate increases.

The Company reviews the individual securities in its portfolio to determine whether a decline in a security’s fair value below the amortized cost basis is other-than-temporary. The Company determined that as of March 31, 2015, there were no investments in its portfolio that were other-than-temporarily impaired.