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Short-term Investments
 9 Months Ended
Sep. 30, 2019
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 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



The Company has classified all of its investment securities as available-for-sale as the sale of such securities may be required prior to maturity to implement management strategies, and accordingly, carries these investments at fair value. Unrealized gains and losses on available-for-sale securities are included as a component of comprehensive loss. At September 30, 2019, 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.