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Marketable Securities (Tables)
 9 Months Ended
Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]  
Summary of Available for Sale Securities Held

The following table summarizes the available-for-sale securities held at September 30, 2018 and December 31, 2017 (in thousands):

 

Description

 

Amortized

cost

 

 

Unrealized

gains

 

 

Unrealized

losses

 

 

Fair

value

 

September 30, 2018

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

U.S. government agency securities and treasuries

 

$

1,042,297

 

 

$

1

 

 

$

(4,705

)

 

$

1,037,593

 

Certificates of deposit

 

 

9,320

 

 

 

—

 

 

 

—

 

 

 

9,320

 

Total

 

$

1,051,617

 

 

$

1

 

 

$

(4,705

)

 

$

1,046,913

 

December 31, 2017

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

U.S. government agency securities and treasuries

 

$

841,895

 

 

$

—

 

 

$

(3,579

)

 

$

838,316

 

Certificates of deposit

 

 

17,480

 

 

 

1

 

 

 

—

 

 

 

17,481

 

Total

 

$

859,375

 

 

$

1

 

 

$

(3,579

)

 

$

855,797