3. Cash Equivalents & Marketable Securities

Cash equivalents and marketable securities consisted of the following at December 31, 2017 (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
December 31, 2017	Cost		Gains		Losses		Value
Cash equivalents:
Money market funds	$	134,635	$	—	$	—	$	134,635
U.S. Treasuries		11,995		—		—		11,995
Marketable securities:
U.S. Treasuries		123,606		—		(47)		123,559
Government agency securities		58,979		—		(29)		58,950
Total cash equivalents and marketable securities	$	329,215	$	—	$	(76)	$	329,139

Cash equivalents and marketable securities consisted of the following at December 31, 2016 (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
December 31, 2016	Cost		Gains		Losses		Value
Cash equivalents:
Money market funds	$	185,323	$	—	$	—	$	185,323
Total cash equivalents and marketable securities	$	185,323	$	—	$	—	$	185,323

At December 31, 2017, the Company held 25 securities that were in an unrealized loss position. The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months at December 31, 2017 was $174.0 million, and there were no securities held by the Company in an unrealized loss position for more than 12 months. As of December 31, 2017, the Company did not intend to sell, and would not be more likely than not required to sell, the securities in an unrealized loss position before recovery of their amortized cost bases. Furthermore, the Company has determined that there was no material change in the credit risk of these securities. As a result, the Company determined it did not hold any securities with any other-than-temporary impairment as of December 31, 2017.

There were no realized gains or losses on available-for-sale securities during the year ended December 31, 2017 and 2016.