Short-term Investments (Tables)

3 Months Ended

Mar. 31, 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 March 31, 2018	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Corporate debt securities	$	34,011	$	—	$	(60	)	$	33,951
U.S. government-related debt securities	10,961		—		(52		)	10,909
Total available-for-sale securities	$	44,972	$	—	$	(112	)	$	44,860

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):

	March 31, 2018		December 31, 2017
Maturing in one year or less	$	44,860	$	39,985
Maturing in one to three years	—		11,434
Total available-for-sale securities	$	44,860	$	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 March 31, 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	$	23,031	$	(60	)	$	—	$	—		$	23,031	$	(60	)
U.S. government-related debt securities	8,910		(51		)	1,999		(1		)	10,909		(52		)
Total	$	31,941	$	(111	)	$	1,999	$	(1	)	$	33,940	$	(112	)