Short-term Investments (Tables)

12 Months Ended

Dec. 31, 2017

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 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

Type of securities as of December 31, 2016	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Corporate debt securities	$	36,198	$	4	$	(42	)	$	36,160
U.S. government-related debt securities	17,312		1		(20		)	17,293
Total available-for-sale securities	$	53,510	$	5	$	(62	)	$	53,453

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

The fair values of available-for-sale securities by contractual maturity at December 31 were as follows (in thousands):

	2017		2016
Maturing in one year or less	$	39,985	$	46,310
Maturing in one to three years	11,434		7,143
Total available-for-sale securities	$	51,419	$	53,453