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Short-term Investments (Tables)
 9 Months Ended
Sep. 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 September 30, 2018
 
Maturity
 
Amortized cost
 
Gross unrealized gains
 
Gross unrealized losses
 
Estimated fair value
Corporate debt securities
1 year or less
 
$
74,366

 
$
1

 
$
(67
)
 
$
74,300

Commercial paper
1 year or less
 
51,612

 
2

 
(12
)
 
51,602

 
 
 
$
125,978

 
$
3

 
$
(79
)
 
$
125,902

 
 
 
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