MARKETABLE SECURITIES

The amortized cost, gross unrealized gains and losses, and fair value of securities held-to-maturity as of September 30, 2018 and December 31, 2017 were as follows (in thousands):



	September 30, 2018
Short-term marketable securities:		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper		$	158,506	$	—	$	—		$	158,506
Corporate bonds		138,141		—		(174		)	137,967
U.S. government bonds		69,241		—		(33		)	69,208
Agency bonds		48,114		—		(67		)	48,047
Total short-term marketable securities		414,002		—		(274		)	413,728
Total marketable securities		$	414,002	$	—	$	(274	)	$	413,728



	December 31, 2017
Short-term marketable securities:	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper	$	138,412	$	1	$	(1	)	$	138,412
Corporate bonds	45,006		—		(41		)	44,965
Agency bonds	78,958		—		(45		)	78,913
Agency discount bonds	$	10,990	$	—	$	(1	)	$	10,989
Total short-term marketable securities	$	273,366	$	1	$	(88	)	$	273,279
Long-term marketable securities:
Corporate bonds	$	25,032	$	—	$	(71	)	$	24,961
Total long-term marketable securities	$	25,032	$	—	$	(71	)	$	24,961
Total marketable securities	$	298,398	$	1	$	(159	)	$	298,240

The following tables present gross unrealized losses and fair values for those securities that were in an unrealized loss position as of September 30, 2018 and December 31, 2017, aggregated by investment category and the length of time that the individual securities have been in a continuous loss position (in thousands):



	September 30, 2018
	Less Than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Corporate bonds	$	132,992	$	(174	)	$	—	$	—	$	132,992	$	(174	)
Agency bonds	48,047		(67		)	—		—		48,047		(67		)
U.S. government bonds	69,208		(33		)	—		—		69,208		(33		)
Commercial paper	—		—			—		—		—		—
Total	$	250,247	$	(274	)	$	—	$	—	$	250,247	$	(274	)



	December 31, 2017
	Less Than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Agency bonds	$	78,913	$	(45	)	$	—	$	—	$	78,913	$	(45	)
Corporate bonds	62,927		(112		)	—		—		62,927		(112		)
Agency discount notes	10,989		(1		)	—		—		10,989		(1		)
Commercial paper	3,975		(1		)	—		—		3,975		(1		)
Total	$	156,804	$	(159	)	$	—	$	—	$	156,804	$	(159	)

The Company periodically reviews its investment portfolio for other-than-temporary impairment. The Company considers such factors as the duration, severity and reason for the decline in value, and the potential recovery period. The Company also considers whether it is more likely than not that it will be required to sell the securities before the recovery of their amortized cost basis, and whether the amortized cost basis cannot be recovered as a result of credit losses. During the three and nine months ended September 30, 2018 and 2017, the Company did not recognize any other-than-temporary impairment loss.