Short-term Investments (Tables)

9 Months Ended

Sep. 30, 2018

Investments, Debt and Equity Securities [Abstract]

Short-Term Investments Available-for-Sale Securities

Short-term investments consisted of available-for-sale securities as follows (in thousands):

Type of securities as of September 30, 2018	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Corporate debt securities	$	48,971	$	—	$	(19	)	$	48,952
U.S. government-related debt securities	17,916		—		(34		)	17,882
Asset-backed securities	6,898		—		(1		)	6,897
Total available-for-sale securities	$	73,785	$	—	$	(54	)	$	73,731

Type of securities as of December 31, 2017	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Corporate debt securities	$	35,567	$	—	$	(53	)	$	35,514
U.S. government-related debt securities	15,951		—		(46		)	15,905
Total available-for-sale securities	$	51,518	$	—	$	(99	)	$	51,419

Fair Values of Available-for-Sale Securities by Contractual Maturity

The fair values of available-for-sale securities by contractual maturity were as follows (in thousands):

	September 30, 2018		December 31, 2017
Maturing in one year or less	$	73,731	$	39,985
Maturing in one to three years	—		11,434
Total available-for-sale securities	$	73,731	$	51,419

Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Table Text Block]

The following table summarizes investments that have been in a continuous unrealized loss position as of September 30, 2018 (in thousands).

	Less Than 12 Months					12 Months or Greater				Total
	Fair value		Gross unrealized losses			Fair value		Gross unrealized losses		Fair value		Gross unrealized losses
Corporate debt securities	$	19,965	$	(19	)	$	—	$	—	$	19,965	$	(19	)
U.S. government-related debt securities	17,883		(34		)	—		—		17,883		(34		)
Asset-backed securities	4,456		(1		)	—		—		4,456		(1		)
Total	$	42,304	$	(54	)	$	—	$	—	$	42,304	$	(54	)