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Short-term Investments
 12 Months Ended
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Short-term Investments
Short-term Investments
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


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


The Company has both the intent and ability to sell its available-for-sale investments maturing greater than one year within 12 months from the balance sheet date and, accordingly, has classified these securities as current in the consolidated balance sheet.
The following table summarizes investments that have been in a continuous unrealized loss position as of December 31, 2017 (in thousands).
 
Less Than 12 Months
 
12 Months or
Greater
 
Total
 
Fair
value
 
Gross
unrealized
losses
 
Fair
value
 
Gross
unrealized
losses
 
Fair
value
 
Gross
unrealized
losses
Corporate debt securities
$
26,857

 
$
(53
)
 
$
—

 
$
—

 
$
26,857

 
$
(53
)
U.S. government-related debt securities
8,911

 
(40
)
 
3,994

 
(6
)
 
12,905

 
(46
)
Total
$
35,768

 
$
(93
)
 
$
3,994

 
$
(6
)
 
$
39,762

 
$
(99
)
The Company invests in securities that are rated investment grade or better. The unrealized losses on investments as of December 31, 2017 and December 31, 2016 were primarily caused by interest rate increases.
The Company reviews the individual securities in its portfolio to determine whether a decline in a security’s fair value below the amortized cost basis is other-than-temporary. The Company determined that as of December 31, 2017, there were no investments in its portfolio that were other-than-temporarily impaired.