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Investments (Tables)
 12 Months Ended
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]  
Summary of investments
The following is a summary of our investments as of December 31, 2016 and 2015 (in thousands):
 
December 31, 2016
 
 
 
Gross
 
Gross
 
Estimated
 
Amortized
 
Unrealized
 
Unrealized
 
Fair
 
Cost
 
Gains
 
Losses
 
Value
Corporate debt securities
    
 
    
 
    
 
    
Due in one year or less
$
106,430

 
$
3

 
$
(69
)
 
$
106,364

Due in one to three years
139,742

 
32

 
(281
)
 
139,493

U.S. treasury and government agency securities
 
 
 
 
 
 
 

Due in one year or less
1,021

 
—

 
—

 
1,021

Due in one to three years
11,395

 
—

 
(52
)
 
11,343

Commercial paper
 
 
 
 
 
 
 

Due in one year or less
40,560

 
—

 
—

 
40,560

Certificates of deposit
 
 
 
 
 
 
 
Due in one year or less
6,000

 
—

 
—

 
6,000

Total investments
$
305,148

 
$
35

 
$
(402
)
 
$
304,781

 
 
December 31, 2015
 
 
 
Gross
 
Gross
 
Estimated
 
Amortized
 
Unrealized
 
Unrealized
 
Fair
 
Cost
 
Gains
 
Losses
 
Value
Corporate debt securities
   
 
    
 
    
 
 
Due in one year or less
$
27,964

 
$
—

 
$
(38
)
 
$
27,926

Due in one to three years
173,652

 
3

 
(904
)
 
172,751

Commercial paper
 

 
 

 
 

 
 

Due in one year or less
34,452

 
2

 
(5
)
 
34,449

Municipal securities
 

 
 

 
 

 
 

Due in one year or less
2,500

 
—

 
—

 
2,500

Total investments
$
238,568

 
$
5

 
$
(947
)
 
$
237,626