3. Available-for-Sale Marketable Securities

As of December 31, 2017, and 2016, the Company’s available-for-sale marketable securities consisted of money market funds and debt securities issued by U.S. government-sponsored entities and by investment grade institutions. As of December 31, 2016, the Company’s available-for-sale marketable securities also included debt securities issued by the U.S. Treasury.

The following tables summarize the Company's available-for-sale marketable securities by major type of security as of December 31, 2017, and 2016:

As of December 31, 2017

			Gross Unrealized					Estimated Fair
Type of Security	Amortized Cost		Gains		Losses			Value
Money market funds	$	39,988	$	—	$	(37	)	$	39,951
U.S. government agency obligations		7,799		—		(5	)		7,794
Corporate bonds		15,919		—		(12	)		15,907
Commercial paper		19,545		—		(16	)		19,529
Total available-for-sale marketable securities	$	83,251	$	—	$	(70	)	$	83,181

As of December 31, 2016

			Gross Unrealized					Estimated Fair
Type of Security	Amortized Cost		Gains		Losses			Value
Money market funds	$	8,268	$	8	$	—		$	8,276
U.S. Treasury securities		2,523		—		(1	)		2,522
U.S. government agency obligations		3,501		1		—			3,502
Corporate bonds		16,683		—		(6	)		16,677
Commercial paper		15,206		3		(2	)		15,207
Total available-for-sale marketable securities	$	46,181	$	12	$	(9	)	$	46,184

All available-for-sale marketable securities are classified in the Company’s Balance Sheets as Marketable securities.

The Company classifies its marketable debt securities based on their contractual maturity dates. As of December 31, 2017, the Company’s marketable debt securities mature at various dates through July 2018. The amortized cost and fair values of marketable debt securities by contractual maturity were as follows. The table does not include money market funds that are classified as available-for sale marketable securities.

	As of December 31,
	2017				2016
Contractual maturity	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than one year	$	43,263	$	43,230	$	37,913	$	37,908

During the years ended December 31, 2017 and 2016, the Company sold shares of its investments in available-for-sale marketable securities with total fair values of $8,755 and $34,003, respectively. The cost of the available-for-sale marketable securities that were sold was determined by specific identification. The sales of the investments in available-for-sale marketable securities during each year resulted in realized gains, totaling $5 and $23, respectively.

The following tables show the fair value of the Company's available-for-sale marketable securities that have unrealized losses and that are deemed to be only temporarily impaired, aggregated by investment category and length of time that the individual investments have been in a continuous unrealized loss position.

	Less than 12 Months					12 Months or Greater				Total
			Gross					Gross				Gross
			Unrealized					Unrealized				Unrealized
As of December 31, 2017	Fair Value		Losses			Fair Value		Losses		Fair Value		Losses
Money market funds	$	39,951	$	(37	)	$	—	$	—	$	39,951	$	(37	)
U.S. government agency obligations		7,794		(5	)		—		—		7,794		(5	)
Corporate bonds		15,907		(12	)		—		—		15,907		(12	)
Commercial paper		19,031		(16	)		—		—		19,031		(16	)
Total	$	82,683	$	(70	)	$	—	$	—	$	82,683	$	(70	)

	Less than 12 Months					12 Months or Greater				Total
			Gross					Gross				Gross
			Unrealized					Unrealized				Unrealized
As of December 31, 2016	Fair Value		Losses			Fair Value		Losses		Fair Value		Losses
U.S. Treasury securities	$	2,522	$	(1	)	$	—	$	—	$	2,522	$	(1	)
Corporate bonds		9,919		(6	)		—		—		9,919		(6	)
Commercial paper		5,227		(2	)		—		—		5,227		(2	)
Total	$	17,668	$	(9	)	$	—	$	—	$	17,668	$	(9	)

As of December 31, 2017, and 2016, the Company held a total of 30 out of 31 positions and 18 out of 34 positions, respectively, that were in an unrealized loss position, none of which had been in an unrealized loss position for 12 months or greater. Based on the Company’s review of these securities, the Company believes that the cost basis of its available-for-sale marketable securities is recoverable and that, therefore, it had no other-than-temporary impairments on these securities as of December 31, 2017, or 2016. The Company does not intend to sell these debt securities before maturity and the Company believes it is not more likely than not that it will be required to sell these securities before the recovery of their amortized cost basis, which may be maturity.