Investments (Tables)

12 Months Ended

Dec. 31, 2017

Available For Sale Securities [Abstract]

Amortized Cost and Fair Value on Available-for-Sale Investments and Unrealized Gains and Losses

The amortized cost and fair value on the available-for-sale investments and unrealized gains and losses as of December 31, 2017 and 2016 were as follows (in thousands):

	December 31, 2017
	Amortized		Gross Unrealized		Gross Unrealized			Fair
	Cost		Gains		Losses			Value
Amounts maturing in one year or less
Corporate notes and obligations	$	38,044	$	—	$	(24	)	$	38,020
U.S. government treasury securities		55,967		—		(86	)		55,881
Total short-term available-for-sale debt securities	$	94,011	$	—	$	(110	)	$	93,901

	December 31, 2016
	Amortized		Gross Unrealized		Gross Unrealized			Fair
	Cost		Gains		Losses			Value
Amounts maturing in one year or less
Corporate notes and obligations	$	29,866	$	—	$	(25	)	$	29,841
U.S. government treasury securities		6,900		—		—			6,900
Total short-term available-for-sale debt securities	$	36,766	$	—	$	(25	)	$	36,741
Amounts maturing in greater than one year
Corporate notes and obligations	$	4,010	$	—	$	(11	)	$	3,999
U.S. government treasury securities		34,505		—		(58	)		34,447
Total long-term available-for-sale debt securities	$	38,515	$	—	$	(69	)	$	38,446

Gross Unrealized Losses and Fair Value for Investments in Unrealized Loss Position

The following table presents gross unrealized losses and fair values for those investments that were in an unrealized loss position as of December 31, 2017, aggregated by investment category and the length of time that individual securities have been in a continuous loss position (in thousands):

	December 31, 2017
	Less than 12 months					12 months or greater
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
Corporate notes and obligations	$	18,873	$	(22	)	$	3,998	$	(3	)
U.S. government treasury securities		24,415		(52	)		31,466		(33	)
Total short-term available-for-sale debt securities	$	43,288	$	(74	)	$	35,464	$	(36	)