Short-Term Investments

6 Months Ended

Jun. 30, 2019

5. Short-Term Investments

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

	June 30, 2019
			Gross		Gross
	Amortized		Unrealized		Unrealized		Estimated
	Cost		Gains		Losses		Fair Value
U.S. treasury bills and government agency obligations	$	99,524	$	91	$	—	$	99,615
U.S. corporate debt securities		33,007		30		—		33,037
Foreign corporate debt securities		10,002		16		—		10,018
U.S. commercial paper		48,969		10		—		48,979
Foreign commercial paper		31,318		—		—		31,318
Total	$	222,820	$	147	$	—	$	222,967

	December 31, 2018
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost		Gains		Losses			Fair Value
U.S. treasury bills and government agency obligations	$	59,747	$	—	$	(6	)	$	59,741
U.S. corporate debt securities		59,164		—		(77	)		59,087
Foreign corporate debt securities		5,041		5		—			5,046
U.S. commercial paper		52,800		—		—			52,800
Foreign commercial paper		123,870		—		(9	)		123,861
Total	$	300,622	$	5	$	(92	)	$	300,535

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, 2019 and 2018.

Realized gains and losses associated with our investments, 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, 2019 and 2018. We recognized $8,000 in realized gains during the six months ended June 30, 2019. We did not recognize any realized gains or losses during the six months ended June 30, 2018.