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Marketable Securities
 12 Months Ended
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Marketable Securities
Marketable Securities
Marketable securities at December 31, 2017 consisted of the following (in thousands):
 
Amortized Cost
 
Unrealized Gains
 
Unrealized Losses
 
Fair Value
Current:
 
 
 
 
 
 
 
Certificates of deposit
$
8,081

 
$
—

 
$
(11
)
 
$
8,070

U.S. Treasuries
113,852

 
—

 
(119
)
 
113,733

Government securities
44,421

 
—

 
(57
)
 
44,364

Corporate debt securities
155,222

 
—

 
(177
)
 
155,045

Non-current:
 
 
 
 
 
 
 
Certificates of deposit
960

 
—

 
(8
)
 
952

U.S. Treasuries
36,165

 
—

 
(311
)
 
35,854

Government securities
23,992

 
—

 
(182
)
 
23,810

Corporate debt securities
83,722

 
—

 
(524
)
 
83,198

 
$
466,415

 
$
—

 
$
(1,389
)
 
$
465,026

Marketable securities at December 31, 2016 consisted of the following (in thousands):
 
Amortized
Cost
 
Unrealized
Gains
 
Unrealized
Losses
 
Fair
Value
Current:







Certificates of deposit
$
11,280


$
2


$
(3
)

$
11,279

U.S. Treasuries
141,678


2


(62
)

141,618

Government securities
19,533


—


(23
)

19,510

Corporate debt securities
208,285


3


(135
)

208,153

Non-current:







Certificates of deposit
7,600


6


(13
)

7,593

Government securities
4,499


—


(21
)

4,478

Corporate debt securities
20,248


—


(69
)

20,179


$
413,123


$
13


$
(326
)

$
412,810


There were no material realized gains or losses on marketable securities for the years ended December 31, 2017 and 2016.
At December 31, 2017 and 2016, we held 240 and 158 debt securities that were in an unrealized loss position for less than one year, respectively. The aggregate fair value of debt securities in an unrealized loss position at December 31, 2017 and 2016 was $439.4 million and $335.4 million, respectively. There were no individual securities that were in a significant unrealized loss position as of December 31, 2017 and 2016. We evaluated our securities for other-than-temporary impairment and considered the decline in market value for the securities to be primarily attributable to current economic and market conditions. It is not more likely than not that we will be required to sell the securities, and we do not intend to do so prior to the recovery of the amortized cost basis. Based on this analysis, these marketable securities were not considered to be other-than-temporarily impaired as of December 31, 2017 and 2016.