4. Short-Term Investments

The following table summarizes the short-term investments securities held at March 31, 2019 and December 31, 2018 (in thousands):


			Gross Unrealized		Gross Unrealized
	Amortized Cost		Gains		Losses		Fair Value
March 31, 2019
Corporate debt securities	$	2,012	$	—	$	—	$	2,012
Reverse repurchase agreements		38,000		—		—		38,000
Total	$	40,012	$	—	$	—	$	40,012


			Gross Unrealized		Gross Unrealized
	Amortized Cost		Gains		Losses		Fair Value
December 31, 2018
Corporate debt securities	$	7,280	$	—	$	(4)	$	7,276
Reverse repurchase agreements		15,000		—		—		15,000
Total	$	22,280	$	—	$	(4)	$	22,276

The contractual maturities of all short-term investments held at March 31, 2019 and December 31, 2018 were one year or less. There was one short-term investment in an unrealized loss position at March 31, 2019 and seven short-term investments in an unrealized loss position at December 31, 2018, none of which had been in an unrealized loss position for more than 12 months. The aggregate fair value of these investments was approximately $2.0 million and $7.3 million at March 31, 2019 and December 31, 2018, respectively. The Company did not hold any investments with other-than-temporary impairments at March 31, 2019 and December 31, 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-month periods ended March 31, 2019 and 2018. All proceeds in the three-month periods ended March 31, 2019 and March 31, 2018 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 months ended March 31, 2019 and 2018.