Note 3. Investments

Available-for-sale investments consist of fixed-income securities that are accounted for at fair value. Premiums and discounts paid on securities at the time of purchase are recorded as accrued interest and amortized over the period of maturity. 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

As interest rates increase, those securities purchased at a lower yield show a mark-to-market unrealized loss. The longer the duration of these securities, the more susceptible they are to changes in market interest rates and bond yields. The unrealized losses are due primarily to changes in interest rates. The Company regularly reviews investments for other-than-temporary impairment using both qualitative and quantitative criteria.

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	)

The Company did not consider any of the unrealized losses on its investments to be other-than-temporarily impaired based on its evaluation of available evidence, which includes the Company’s intent as of December 31, 2017 to hold these investments until the cost basis is recovered. Realized gains and losses on sales of available-for-sale securities were immaterial for all periods presented.