Short-term Investments

6 Months Ended

Jun. 30, 2020

5. Short-term Investments

The following is a summary of our short-term investments (in thousands):

	June 30, 2020
			Gross		Gross
	Amortized		Unrealized		Unrealized		Estimated
	Cost		Gains		Losses		Fair Value
U.S. treasury bills and government agency obligations	$	100,309	$	560	$	—	$	100,869
U.S. corporate debt securities		44,861		231		—		45,092
Foreign corporate debt securities		23,574		162		—		23,736
U.S. commercial paper		18,968		—		—		18,968
Foreign commercial paper		31,449		—		—		31,449
Total	$	219,161	$	953	$	—	$	220,114

	December 31, 2019
			Gross		Gross
	Amortized		Unrealized		Unrealized		Estimated
	Cost		Gains		Losses		Fair Value
U.S. treasury bills and government agency obligations	$	140,567	$	59	$	—	$	140,626
U.S. corporate debt securities		80,159		11		—		80,170
Foreign corporate debt securities		23,188		15		—		23,203
U.S. commercial paper		32,801		—		—		32,801
Foreign commercial paper		42,274		—		—		42,274
Total	$	318,989	$	85	$	—	$	319,074

The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. We regularly monitor and evaluate the realizable value of our marketable securities. We did not recognize any impairment losses during the three and six months ended June 30, 2020 and 2019.

Realized gains and losses associated with our available-for-sale securities, if any, are reported in the statements of operations and comprehensive loss. We did not recognize any realized gains or losses during the three months ended June 30, 2020 and 2019. We did not recognize any realized gains or losses during the six months ended June 30, 2020. We recognized $8,000 of realized gains during the six months ended June 30, 2019.