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Short-term Investments (Tables)
 9 Months Ended
Sep. 30, 2019
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, 2019
 
Maturity
 
Amortized cost
 
Gross unrealized gains
 
Gross unrealized losses
 
Estimated fair value
Corporate debt securities
2 years or less
 
$
180,580

 
$
271

 
$
(3
)
 
$
180,848

Commercial paper
1 year or less
 
110,918

 
67

 
—

 
110,985

U.S. Agency bonds
2 years or less
 
37,500

 
21

 
—

 
37,521

U.S. Treasury bills
1 year or less
 
32,943

 
21

 
(10
)
 
32,954

 
 
 
$
361,941

 
$
380

 
$
(13
)
 
$
362,308

 
 
 
As of December 31, 2018
 
Maturity
 
Amortized cost
 
Gross unrealized gains
 
Gross unrealized losses
 
Estimated fair value
Corporate debt securities
1 year or less
 
$
111,933

 
$
26

 
$
(43
)
 
$
111,916

Commercial paper
1 year or less
 
74,433

 
—

 
(24
)
 
74,409

U.S. Treasury bills
1 year or less
 
3,771

 
—

 
—

 
3,771

 
 
 
$
190,137

 
$
26

 
$
(67
)
 
$
190,096