Short-term Investments

9 Months Ended

Sep. 30, 2018

Investments, Debt and Equity Securities [Abstract]

Short-term Investments

Short-term Investments

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

Type of securities as of September 30, 2018	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Corporate debt securities	$	48,971	$	—	$	(19	)	$	48,952
U.S. government-related debt securities	17,916		—		(34		)	17,882
Asset-backed securities	6,898		—		(1		)	6,897
Total available-for-sale securities	$	73,785	$	—	$	(54	)	$	73,731

Type of securities as of December 31, 2017	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Corporate debt securities	$	35,567	$	—	$	(53	)	$	35,514
U.S. government-related debt securities	15,951		—		(46		)	15,905
Total available-for-sale securities	$	51,518	$	—	$	(99	)	$	51,419

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

	September 30, 2018		December 31, 2017
Maturing in one year or less	$	73,731	$	39,985
Maturing in one to three years	—		11,434
Total available-for-sale securities	$	73,731	$	51,419

The Company has the ability to sell its available-for-sale investments maturing greater than one year within 12 months from the balance sheet date and, accordingly, has classified these securities as current in the condensed consolidated balance sheets.

The following table summarizes investments that have been in a continuous unrealized loss position as of September 30, 2018 (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	$	19,965	$	(19	)	$	—	$	—	$	19,965	$	(19	)
U.S. government-related debt securities	17,883		(34		)	—		—		17,883		(34		)
Asset-backed securities	4,456		(1		)	—		—		4,456		(1		)
Total	$	42,304	$	(54	)	$	—	$	—	$	42,304	$	(54	)

The Company invests in securities that are rated investment grade or better. The unrealized losses on investments as of September 30, 2018 and December 31, 2017 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 September 30, 2018, there were no investments in its portfolio that were other-than-temporarily impaired.