Marketable Securities

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Marketable Securities

4. Marketable Securities

The amortized cost, unrealized gains and losses and estimated fair value of the Company’s held-to-maturity marketable securities as of December 31, 2017 and 2016 were as follows:

	December 31, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents
Commercial paper	$	39,979	$	—	$	(43	)	$	39,936
Corporate bonds		1,250		—		—			1,250
Short-term investments
Commercial paper		21,480		—		(99	)		21,381
Corporate bonds		2,125		—		(1	)		2,124
Total	$	64,834	$	—	$	(143	)	$	64,691

	December 31, 2016
	Amortized Cost		Unrealized Gains		Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents
Commercial paper	$	59,175	$	2	$	(28	)	$	59,149
Corporate bonds		5,000		1		—			5,001
U.S. government agency bonds		5,500		—		—			5,500
Short term investments
Commercial paper		73,002		—		(214	)		72,788
Corporate bonds		11,089		4		(5	)		11,088
Total	$	153,766	$	7	$	(247	)	$	153,526

All of the Company’s marketable securities were classified as held-to-maturity investments and have maturities within one year of December 31, 2017. Approximately $80 million of the Company’s marketable securities matured during the year ended December 31, 2017, which was invested in money market funds as of December 31, 2017. See Note 14, Fair Value Measurement, for additional details.

The gross unrealized losses, estimated fair value and length of time the individual marketable securities were in a continuous loss position for those marketable securities in an unrealized loss position as of December 31, 2017 and 2016 were as follows:

	December 31, 2017
	Less Than 12 Months					12 Months or Greater				Total
	Estimated Fair Value		Unrealized Loss			Estimated Fair Value		UnrealizedLoss		Estimated Fair Value		Unrealized Loss
	(in thousands)
Commercial paper	$	61,317	$	(142	)	$	—	$	—	$	61,317	$	(142	)
Corporate bonds		3,374		(1	)		—		—		3,374		(1	)
Total	$	64,691	$	(143	)	$	—	$	—	$	64,691	$	(143	)

	December 31, 2016
	Less Than 12 Months					12 Months or Greater				Total
	Estimated Fair Value		Unrealized Loss			Estimated Fair Value		UnrealizedLoss		Estimated Fair Value		Unrealized Loss
	(in thousands)
Commercial paper	$	130,938	$	(242	)	$	—	$	—	$	130,938		$	(242	)
Corporate bonds		6,556		(5	)		—		—		6,556			(5	)
Total	$	137,494	$	(247	)	$	—	$	—	$	137,494		$	(247	)

The Company recognized interest income during the years ended December 31, 2017, 2016 and 2015 of $2.0 million, $1.3 million and $0.5 million, respectively, within net interest (income) expense on the consolidated statements of operations. During the years ended December 31, 2017, 2016 and 2015, the Company did not recognize any other-than-temporary impairment losses related to its marketable securities.

The Company’s marketable securities are classified within Level 2 of the fair value hierarchy (see Note 14, Fair Value Measurement, for further details).