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Short-term Investments
 6 Months Ended
Jun. 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 June 30, 2019
 
Maturity
 
Amortized cost
 
Gross unrealized gains
 
Gross unrealized losses
 
Estimated fair value
Corporate debt securities
2 years or less
 
$
116,958

 
$
156

 
$
(5
)
 
$
117,109

Commercial paper
1 year or less
 
89,293

 
93

 
—

 
89,386

U.S. Treasury bills
1 year or less
 
18,921

 
25

 
—

 
18,946

U.S. Agency bonds
2 years or less
 
6,000

 
—

 
(2
)
 
5,998

Other sovereign securities
1 year or less
 
1,995

 
1

 
—

 
1,996

 
 
 
$
233,167

 
$
275

 
$
(7
)
 
$
233,435

 
 
 
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 June 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.