4.Short-Term Investments

As of December 31, 2017, the Company held no short-term investments. The following table summarizes the short-term investments securities held at September 30, 2018 (in thousands):

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
September 30, 2018
Corporate debt securities	$	10,368	$	—	$	(5	)	$	10,363
U.S reverse repurchase agreements	13,000		—		—			13,000
Total	$	23,368	$	—	$	(5	)	$	23,363

The contractual maturities of all short-term investments held at September 30, 2018 were one year or less. There were ten short-term investments in an unrealized loss position at September 30, 2018, none of which had been in an unrealized loss position for more than 12 months. The aggregate fair value of these investments at September 30, 2018 was approximately $9.4 million. The Company did not hold any investments with other-than-temporary impairment at September 30, 2018.

Gross realized gains and losses on the sales of short-term investments are included in other income, net. Unrealized holding gains or losses for the period that have been included in accumulated other comprehensive income, as well as gains and losses reclassified out of accumulated other comprehensive income into other income, net were not material to the Company’s condensed consolidated results of operations. The cost of investments sold or the amount reclassified out of the accumulated other comprehensive income into other income, net is based on the specific identification method for purposes of recording realized gains and losses. There were no proceeds from sales of short-term investments in the three and nine-month periods ended September 30, 2018 and 2017. All proceeds in the three and nine-month periods ended September 30, 2018 and the nine-month period ended September 30, 2017 related to maturities of underlying investments. The gains on proceeds from maturities of short-term investments were not material to the Company’s condensed consolidated results of operations for the three or nine months periods ended September 30, 2018 and 2017.