3. Available-for-Sale Marketable Securities

As of September 30, 2019 and December 31, 2018, the Company’s available-for-sale marketable securities consisted of debt securities issued by the U.S. Treasury, U.S. government-sponsored entities and investment grade institutions as well as municipal bonds.

The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of September 30, 2019 and December 31, 2018:

As of September 30, 2019


			Gross Unrealized				Estimated Fair
Type of Security	Amortized Cost		Gains		Losses		Value
U.S. Treasury securities	$	12,842	$	27	$	(9)	$	12,860
U.S. government agency obligations		22,426		12		(11)		22,427
Corporate bonds		108,108		145		(27)		108,226
Commercial paper		65,340		19		(3)		65,356
Municipal bonds		5,500		—		—		5,500
Total available-for-sale marketable securities	$	214,216	$	203	$	(50)	$	214,369

As of December 31, 2018


			Gross Unrealized				Estimated Fair
Type of Security	Amortized Cost		Gains		Losses		Value
U.S. Treasury securities	$	19,540	$	—	$	(1)	$	19,539
U.S. government agency obligations		17,860		—		(1)		17,859
Corporate bonds		75,999		5		(94)		75,910
Commercial paper		50,413		—		(23)		50,390
Municipal bonds		4,000		—		—		4,000
Total available-for-sale marketable securities	$	167,812	$	5	$	(119)	$	167,698

All available-for-sale marketable securities are classified as Marketable securities, current or Marketable securities, non-current depending on the contractual maturity date of the individual available-for-sale security.

The Company classifies its marketable debt securities based on their contractual maturity dates. As of September 30, 2019, the Company’s marketable debt securities mature at various dates through September 2021. The amortized cost and fair values of marketable debt securities by contractual maturity were as follows.


	As of September 30, 2019				As of December 31, 2018
Contractual maturity	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than one year	$	158,274	$	158,384	$	146,363		146,302
One year to two years		55,942		55,985		21,449		21,396
Total	$	214,216	$	214,369	$	167,812	$	167,698

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, 2019


	Less than 12 Months				12 Months or Greater				Total
			Gross				Gross				Gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
	Value		Losses		Value		Losses		Value		Losses
U.S. Treasury securities	$	3,177	$	(9)	$	—	$	—	$	3,177	$	(9)
U.S. government agency obligations		5,487		(11)		—		—		5,487		(11)
Corporate bonds		27,689		(27)		—		—		27,689		(27)
Commercial paper		11,906		(3)		—		—		11,906		(3)
Total	$	48,259	$	(50)	$	—	$	—	$	48,259	$	(50)

As of December 31, 2018


	Less than 12 Months				12 Months or Greater				Total
			Gross				Gross				Gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
	Value		Losses		Value		Losses		Value		Losses
U.S. Treasury securities	$	16,392	$	(1)	$	—	$	—	$	16,392	$	(1)
U.S. government agency obligations		5,596		(1)		—		—		5,596		(1)
Corporate bonds		71,322		(94)		—		—		71,322		(94)
Commercial paper		39,445		(23)		—		—		39,445		(23)
Total	$	132,755	$	(119)	$	—	$	—	$	132,755	$	(119)

As of September 30, 2019 and December 31, 2018, the Company held a total of 23 out of 84 positions and 69 out of 84 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, 2019 and December 31, 2018. 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.