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Investment Securities (Tables)
 12 Months Ended
Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]  
Schedule of Investment Securities Classified as Available-for-Sale

Investment securities, all classified as available-for-sale, consisted of the following (in thousands):

 

 

 

December 31, 2017

 

 

 

Amortized

Cost

 

 

Unrealized

Gains

 

 

Unrealized

Losses

 

 

Estimated

Fair

Value

 

U.S. Treasury notes

 

$

32,976

 

 

$

—

 

 

$

(12

)

 

$

32,964

 

Government sponsored enterprise securities

 

 

10,082

 

 

 

—

 

 

 

(10

)

 

 

10,072

 

Corporate debt securities

 

 

138,650

 

 

 

1

 

 

 

(321

)

 

 

138,330

 

Commercial paper

 

 

90,623

 

 

 

—

 

 

 

(65

)

 

 

90,558

 

 

 

$

272,331

 

 

$

1

 

 

$

(408

)

 

$

271,924

 

 

 

 

December 31, 2016

 

 

 

Amortized

Cost

 

 

Unrealized

Gains

 

 

Unrealized

Losses

 

 

Estimated

Fair

Value

 

U.S. Treasury notes

 

$

82,484

 

 

$

6

 

 

$

(3

)

 

$

82,487

 

Government sponsored enterprise securities

 

 

73,789

 

 

 

1

 

 

 

(5

)

 

 

73,785

 

Corporate debt securities

 

 

79,190

 

 

 

—

 

 

 

(72

)

 

 

79,118

 

Commercial paper

 

 

129,861

 

 

 

165

 

 

 

—

 

 

 

130,026

 

 

 

$

365,324

 

 

$

172

 

 

$

(80

)

 

$

365,416