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Short-term Investments (Tables)
 6 Months Ended
Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]  
Schedule of Available-for-sale Securities Reconciliation
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.