The following tables summarize the available-for-sale securities held at December 31, 2018 and 2017 (in thousands):

December 31, 2018	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair Value
Corporate debt securities	$	107,571	$	4	$	(70	)	$	107,505
U.S. government agency securities	$	3,971	$	1	$	—		$	3,972
Total	$	111,542	$	5	$	(70	)	$	111,477

December 31, 2017	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair Value
Corporate debt securities	$	28,593	$	1	$	(9	)	$	28,585
Total	$	28,593	$	1	$	(9	)	$	28,585

The contractual maturity of all securities held at December 31, 2018 was one year or less. There were 37 investments in an unrealized loss position at December 31, 2018, none of which had been in an unrealized loss position for more than twelve months. The aggregate fair value of the securities in an unrealized loss position at December 31, 2018 and 2017 was $96.5 million and $19.3 million, respectively. The Company reviews its investments for other-than-temporary impairment whenever the fair value of an investment is less than amortized cost and evidence indicates that an investment’s carrying amount is not recoverable within a reasonable period of time. To determine whether an impairment is other-than-temporary, the Company considers whether it has the ability and intent to hold the investment until a market price recovery and considers whether evidence indicating the cost of the investment is recoverable outweighs evidence to the contrary. The Company did not hold any securities with an other-than-temporary impairment at December 31, 2018.

Gross realized gains and losses on the sales of investments have not been material to the Company’s consolidated statement of operations.