3. Marketable securities

The following table summarizes the available-for-sale securities held at September 30, 2018 and December 31, 2017 (in thousands):

Description	Amortized cost		Unrealized gains		Unrealized losses			Fair value
September 30, 2018
U.S. government agency securities and treasuries	$	1,042,297	$	1	$	(4,705	)	$	1,037,593
Certificates of deposit		9,320		—		—			9,320
Total	$	1,051,617	$	1	$	(4,705	)	$	1,046,913
December 31, 2017
U.S. government agency securities and treasuries	$	841,895	$	—	$	(3,579	)	$	838,316
Certificates of deposit		17,480		1		—			17,481
Total	$	859,375	$	1	$	(3,579	)	$	855,797

The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At September 30, 2018 and December 31, 2017, the balance in the Company’s accumulated other comprehensive loss was composed primarily of activity related to the Company’s available-for-sale marketable securities. There were no material realized gains or losses recognized on the sale or maturity of available-for-sale securities during the nine months ended September 30, 2018 or 2017, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same periods.

The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of September 30, 2018 and December 31, 2017 was $640.1 million and $704.1 million, respectively. As of September 30, 2018 and December 31, 2017, there were $392.5 million and $134.4 million in securities held by the Company in an unrealized loss position for more than twelve months, respectively. The aggregate unrealized loss on securities held by the Company for less than twelve months as of September 30, 2018 and December 31, 2017 was $1.8 million and $3.3 million, respectively. The aggregate unrealized loss on securities held by the Company for more than twelve months as of September 30, 2018 and December 31, 2017 was $2.9 million and $0.3 million, respectively. The Company has the intent and ability to hold such securities until recovery. The Company determined that there was no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of September 30, 2018 and December 31, 2017.

No available-for-sale securities held as of September 30, 2018 or December 31, 2017 had remaining maturities greater than three years.