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Short-term Investments
 6 Months Ended
Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]  
Short-term Investments
Short-term Investments

The following tables summarize the Company's short-term investments (dollars in thousands):
 
 
 
As of June 30, 2018
 
Maturity
 
Amortized cost
 
Gross unrealized gains
 
Gross unrealized losses
 
Estimated fair value
Corporate debt securities
1 year or less1
 
$
89,084

 
$
—

 
$
(163
)
 
$
88,921

Commercial paper
1 year or less
 
65,313

 
14

 
(27
)
 
65,300

 
 
 
$
154,397

 
$
14

 
$
(190
)
 
$
154,221

 
 
 
As of December 31, 2017
 
Maturity
 
Amortized cost
 
Gross unrealized gains
 
Gross unrealized losses
 
Estimated fair value
Corporate debt securities
1 year or less
 
$
24,264

 
$
—

 
$
(23
)
 
$
24,241

Commercial paper
1 year or less
 
18,912

 
—

 
(19
)
 
18,893

 
 
 
$
43,176

 
$
—

 
$
(42
)
 
$
43,134



1Includes $1.7 million of assets that will mature in 13 months.

Unrealized gains and losses on available-for-sale securities are included as a component of comprehensive loss. At June 30, 2018, the Company did not have any securities in material unrealized loss positions. The Company reviews its investments to identify and evaluate investments that have an indication of possible other-than-temporary impairment. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition and near-term prospects of the investee, and the Company’s intent and ability to hold the investment for a period of time sufficient to allow for any anticipated recovery in market value. The Company does not intend to sell any investments prior to recovery of their amortized cost basis for any investments in an unrealized loss position.