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Marketable Securities
 3 Months Ended
Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]  
Marketable Securities
Marketable Securities
Available-for-sale marketable securities as of March 31, 2018 and December 31, 2017 were as follows (in thousands):
 
March 31, 2018
 
Amortized
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Fair
Value
U.S. Treasury securities
$
3,999

 
$
—

 
$
(2
)
 
$
3,997

Corporate debt securities
55,657

 
4

 
(25
)
 
55,636

Total
$
59,656

 
$
4

 
$
(27
)
 
$
59,633

 
December 31, 2017
 
Amortized
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Fair
Value
U.S. Treasury securities
$
3,995

 
$
—

 
$
(6
)
 
$
3,989

Government-sponsored enterprises
11,998

 
—

 
(7
)
 
11,991

Corporate debt securities
77,462

 
2

 
(50
)
 
77,414

Total
$
93,455

 
$
2

 
$
(63
)
 
$
93,394


All available-for-sale marketable securities held as of March 31, 2018 had contractual maturities of less than one year. All of the Company's available-for-sale marketable securities in an unrealized loss position as of March 31, 2018 and December 31, 2017 were in a loss position for less than twelve months.  There were no unrealized losses at March 31, 2018 or December 31, 2017 that the Company determined to be other-than-temporary.