Note 3. Investments

Available-for-sale securities consist of fixed-income securities that are accounted for at fair value. Available-for-sale securities with an original maturity of 90 days or less are classified within cash and cash equivalents in the condensed consolidated balance sheets. 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 March 31, 2018 and December 31, 2017 were as follows (in thousands):

	March 31, 2018
	Amortized		Gross Unrealized		Gross Unrealized			Fair
	Cost		Gains		Losses			Value
Amounts maturing in one year or less
Corporate notes and obligations	$	23,486	$	—	$	(17	)	$	23,469
U.S. government treasury securities		31,968		1		(44	)		31,925
Total short-term available-for-sale debt securities	$	55,454	$	1	$	(61	)	$	55,394

Amounts maturing in greater than one year
Corporate notes and obligations	$	7,992	$	2	$	(15	)	$	7,979
U.S. government treasury securities		—		—		—			—
Total short-term available-for-sale debt securities	$	7,992	$	2	$	(15	)	$	7,979

	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

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 March 31, 2018 aggregated by investment category and the length of time that individual securities have been in a continuous loss position (in thousands):

	March 31, 2018
	Less than 12 months					12 months or greater
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss

Corporate notes and obligations	$	18,623	$	(32	)	$	—	$	—
U.S. government treasury securities		12,449		(32	)		16,488		(11	)
Total available-for-sale debt securities	$	31,072	$	(64	)	$	16,488	$	(11	)

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 March 31, 2018 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.