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Marketable Securities
 6 Months Ended
Jun. 30, 2017
Investments Debt And Equity Securities [Abstract]  
Marketable securities

3. Marketable securities

The following table summarizes the available-for-sale securities held at June 30, 2017 and December 31, 2016 (in thousands):

 

Description

 

Amortized

Cost

 

 

Unrealized

Gains

 

 

Unrealized

Losses

 

 

Fair

Value

 

June 30, 2017

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

U.S. government agency securities and treasuries

 

$

508,141

 

 

$

—

 

 

$

(780

)

 

$

507,361

 

Certificates of deposit

 

 

3,360

 

 

 

4

 

 

 

—

 

 

 

3,364

 

Total

 

$

511,501

 

 

$

4

 

 

$

(780

)

 

$

510,725

 

December 31, 2016

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

U.S. government agency securities and treasuries

 

$

600,001

 

 

$

34

 

 

$

(575

)

 

$

599,460

 

Certificates of deposit

 

 

6,480

 

 

 

6

 

 

 

(3

)

 

 

6,483

 

Total

 

$

606,481

 

 

$

40

 

 

$

(578

)

 

$

605,943

 

 

No available-for-sale securities held as of June 30, 2017 or December 31, 2016 had remaining maturities greater than three years.