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Short-term Investments (Tables)
3 Months Ended
Mar. 31, 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 March 31, 2018
 
Maturity
 
Amortized cost
 
Gross unrealized gains
 
Gross unrealized losses
 
Estimated fair value
Corporate debt securities
2 year or less
 
$
86,479

 
$

 
$
(260
)
 
$
86,219

Commercial paper
1 year or less
 
41,936

 

 
(70
)
 
41,866

 
 
 
$
128,415

 
$

 
$
(330
)
 
$
128,085

 
 
 
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