Note 2. Short-Term Investments

At January 31, 2018, short-term investments consisted of the following (in thousands):

			Gross		Gross			Estimated
	Amortized		unrealized		unrealized			fair
	cost		gains		losses			value
Available-for-sale securities:
Asset-backed securities	$	67,875	$	—	$	(424	)	$	67,451
Commercial paper		19,926		—		(12	)		19,914
Corporate notes and bonds		160,499		1		(759	)		159,741
Foreign government bonds		1,504		—		(18	)		1,486
Mortgage backed securities		11,555		—		(75	)		11,480
U.S. agency obligations		71,206		1		(76	)		71,131
U.S. treasury securities		110,707		5		(136	)		110,576
Total available-for-sale securities	$	443,272	$	7	$	(1,500	)	$	441,779

At January 31, 2017, short-term investments consisted of the following (in thousands):

			Gross		Gross			Estimated
	Amortized		unrealized		unrealized			fair
	cost		gains		losses			value
Available-for-sale securities:
Asset-backed securities	$	20,729	$	5	$	(16	)	$	20,718
Commercial paper		20,567		4		(1	)		20,570
Corporate notes and bonds		92,843		14		(101	)		92,756
U.S. agency obligations		87,091		12		(51	)		87,052
U.S. treasury securities		80,277		4		(111	)		80,170
Total available-for-sale securities	$	301,507	$	39	$	(280	)	$	301,266

The following table summarizes the estimated fair value of our short-term investments, designated as available-for-sale and classified by the contractual maturity date of the securities as of the dates shown (in thousands):

	January 31,
	2018		2017
Due in one year or less	$	308,172	$	225,183
Due in greater than one year		133,607		76,083
Total	$	441,779	$	301,266

We have certain available-for-sale securities in a gross unrealized loss position, some of which have been in that position for more than 12 months. We review our debt securities classified as short-term investments on a regular basis to evaluate whether or not any security has experienced an other-than-temporary decline in fair value. We consider factors such as the length of time and extent to which the market value has been less than the cost, our financial position and near-term prospects and our intent to sell, or whether it is more likely than not we will be required to sell the investment before recovery of the investment’s amortized-cost basis. If we determine that an other-than-temporary decline exists in one of these securities, we would write down the respective investment to fair value. For debt securities, the portion of the write-down related to credit loss would be recognized as other income, net in our consolidated statements of comprehensive income. Any portion not related to credit loss would be included in accumulated other comprehensive income (loss). There were no impairments considered other-than-temporary as of January 31, 2018 and 2017.

The following table shows the fair values and the gross unrealized losses of these available-for-sale securities aggregated by investment category as of January 31, 2018 (in thousands):

			Gross
	Fair		unrealized
	value		losses
Asset-backed securities	$	65,690	$	(424	)
Commercial paper		19,914		(12	)
Corporate notes and bonds		155,419		(759	)
Foreign government bonds		1,485		(18	)
Mortgage backed securities		11,481		(75	)
U.S. agency obligations		66,655		(76	)
U.S. treasury securities		82,147		(136	)

The following table shows the fair values and the gross unrealized losses of these available-for-sale securities aggregated by investment category as of January 31, 2017 (in thousands):

			Gross
	Fair		unrealized
	value		losses
Asset-backed securities	$	14,027	$	(16	)
Commercial paper		5,694		(1	)
Corporate notes and bonds		67,220		(101	)
U.S. agency obligations		40,549		(51	)
U.S. treasury securities		68,704		(111	)