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Cash and Cash Equivalents and Investments
 6 Months Ended
Jun. 30, 2018
Cash and Cash Equivalents and Investments [Abstract]  
Cash and Cash Equivalents Disclosure
Cash and Cash Equivalents and Investments
 
The fair value of cash and cash equivalents and investments held at June 30, 2018 and December 31, 2017 are as follows: 
 
 
As of June 30, 2018
 
 
Amortized Cost
 
Gross Unrealized Gains
 
Gross Unrealized Losses
 
Estimated Fair Value
 
 
(in thousands)
Cash and cash equivalents
 
$
15,044

 
$
—

 
$
—

 
$
15,044

Securities maturing within one year:
 
 
 
 
 
 
 
 
U.S. treasury securities
 
174,262

 
—

 
(180
)
 
174,082

Corporate debt securities
 
20,601

 
—

 
(38
)
 
20,563

Total short-term investments
 
$
194,863

 
$
—

 
$
(218
)
 
$
194,645

Total cash and cash equivalents and investments
 
$
209,907

 
$
—

 
$
(218
)
 
$
209,689

 
 
 
 
 
 
 
 
 
 
 
As of December 31, 2017
 
 
Amortized Cost
 
Gross Unrealized Gains
 
Gross Unrealized Losses
 
Estimated Fair Value
 
 
 
 
(in thousands)
 
 
Cash and cash equivalents
 
$
61,661

 
$
—

 
$
—

 
$
61,661

Securities maturing within one year:
 
 
 
 
 
 
 
 
U.S. treasury securities
 
222,316

 
—

 
(168
)
 
222,148

Corporate debt securities
 
27,033

 
—

 
(54
)
 
26,979

Total short-term investments
 
$
249,349

 
$
—

 
$
(222
)
 
$
249,127

Total cash and cash equivalents and investments
 
$
311,010

 
$
—

 
$
(222
)
 
$
310,788



There were no realized losses for the six months ended June 30, 2018, and $7,000 in realized losses for the six months ended June 30, 2017. The cost of securities sold is based on the specific identification method.