3. Available-for-Sale Marketable Securities

As of September 30, 2018 and December 31, 2017, the Company’s available-for-sale marketable securities consisted of money market funds and debt securities issued by the U.S. Treasury, U.S. government-sponsored entities and by investment grade institutions.

The following tables summarize the Company's available-for-sale marketable securities by major type of security as of September 30, 2018 and December 31, 2017:

As of September 30, 2018

			Gross Unrealized
Type of Security	Amortized Cost		Gains		Losses			Estimated Fair Value
Money market funds	$	51,428	$	27	$	—		$	51,455
U.S. Treasury securities		2,996		—		—			2,996
U.S. government agency obligations		2,592		—		—			2,592
Corporate bonds		10,933		1		(5	)		10,929
Commercial paper		41,386		1		(11	)		41,376
Total available-for-sale marketable securities	$	109,335	$	29	$	(16	)	$	109,348

As of December 31, 2017

			Gross Unrealized
Type of Security	Amortized Cost		Gains		Losses			Estimated Fair 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

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

The Company classifies its marketable debt securities based on their contractual maturity dates. As of September 30, 2018, the Company’s marketable debt securities mature at various dates through April 2019. 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 September 30, 2018

As of December 31, 2017

Contractual maturity

Amortized

Cost

Fair

Value

Amortized

Cost

Fair

Value

Less than one year

57,907

57,893

43,263

43,230

During the nine months ended September 30, 2018, the Company sold shares of a money market fund, that is classified as an available-for-sale marketable security, with a total fair value of $28,250. The cost of the money market fund shares that were sold was determined by specific identification. The sales of the shares of the money market fund resulted in a realized loss of $32.

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.

As of September 30, 2018

	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 bonds	$	10,929	$	(5	)	$	—	$	—	$	10,929	$	(5	)
Commercial paper		41,376		(11	)		—		—		41,376		(11	)
Total	$	52,305	$	(16	)	$	—	$	—	$	52,305	$	(16	)

As of December 31, 2017

	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses		Fair Value		Gross Unrealized 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	)

As of September 30, 2018 and December 31, 2017, the Company held a total of 26 out of 31 positions and 30 out of 31 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 September 30, 2018 and December 31, 2017. 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.