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Cash Equivalents and Investments (Tables)
12 Months Ended
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Available-for-sale securities
The following table is a summary of amortized cost, unrealized gain and loss, and fair value (in thousands):
 
December 31, 2017
 
December 31, 2016
 
Cost
 
Unrealized
 
Fair Value
 
Cost
 
Unrealized
 
Fair Value
 
 
Gains
 
Losses
 
 
 
Gains
 
Losses
 
Money market funds
$
236,744

 
$

 
$

 
$
236,744

 
$
60,639

 
$

 
$

 
$
60,639

U.S. treasury securities

 

 

 

 
81,103

 
4

 
(28
)
 
81,079

U.S. government agency obligations

 

 

 

 
40,968

 
1

 
(22
)
 
40,947

Total cash equivalents and available-for-sale securities
$
236,744

 
$

 
$

 
$
236,744

 
$
182,710

 
$
5

 
$
(50
)
 
$
182,665

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Classified as:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Cash equivalents
 
 
 
 
 
 
$
236,744

 
 
 
 
 
 
 
$
60,639

Short-term investments
 
 
 
 
 
 

 
 
 
 
 
 
 
122,026

Total cash equivalents and available-for-sale securities
 
 
 
 
 
 
$
236,744

 
 
 
 
 
 
 
$
182,665