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Marketable Securities
 9 Months Ended
Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]  
Marketable Securities Marketable Securities
At September 30, 2017, marketable securities consisted of the following (in thousands):

 Amortized Cost  Unrealized Gains  Unrealized Losses  Aggregate Fair Value 
U.S. treasury debt securities $2,888  $—  $(3) $2,885 
U.S. corporate debt securities 12,161  1  (14) 12,148 
Money market funds 52,027  —  —  52,027 
 $67,076  $1  $(17) $67,060 
Included in cash and cash equivalents $52,027  $—  $—  $52,027 
Included in marketable securities $15,049  $1  $(17) $15,033 
At December 31, 2016, marketable securities consisted of the following (in thousands):

 Amortized Cost  Unrealized Gains  Unrealized Losses  Aggregate Fair Value 
U.S. treasury debt securities $3,503  $—  $(5) $3,498 
U.S. corporate debt securities 7,943  1  (7) 7,937 
Money market funds 43,496  —  —  43,496 
 $54,942  $1  $(12) $54,931 
Included in cash and cash equivalents $43,496  $—  $—  $43,496 
Included in marketable securities $11,446  $1  $(12) $11,435 

The following table presents gross unrealized losses and fair values for those marketable securities that were in an unrealized loss position as of September 30, 2017, aggregated by investment category and the length of time that individual securities have been in a continuous loss position (in thousands):

 As of September 30, 2017 
 Less than 12 months  12 months or greater 
 Fair Value  Unrealized Loss  Fair Value  Unrealized Loss 
U.S. treasury debt securities $2,885  $(3) $—  $— 
U.S. corporate debt securities 9,884  (14) —  — 
Total $12,769  $(17) $—  $— 

We do not believe any of the unrealized losses represented an other-than-temporary impairment based on our evaluation of available evidence, which includes our intent as of September 30, 2017 to hold these investments until the cost basis is recovered.