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Investments and Fair Value Measurements (Tables)
 6 Months Ended
Jun. 30, 2018
Investments and Fair Value Measurements [Abstract]  
Summary of Investments
The following is a summary of our investments at June 30, 2018 and December 31, 2017 (in thousands):

     
Gross Unrealized
  
Estimated
 
June 30, 2018
 
Cost
  
Gains
  
Losses
  
Fair Value
 
Available-for-sale securities:
            
Corporate debt securities
 
$
82,414
  
$
—
  
$
(127
)
 
$
82,287
 
Debt securities issued by U.S. government agencies
  
81,893
   
3
   
(98
)
  
81,798
 
Total securities with a maturity of one year or less
  
164,307
   
3
   
(225
)
  
164,085
 
                 
Corporate debt securities
  
935
   
—
   
(9
)
  
926
 
Debt securities issued by U.S. government agencies
  
—
   
—
   
—
   
—
 
Total securities with a maturity of one to two years
  
935
   
—
   
(9
)
  
926
 
Total available-for-sale securities
 
$
165,242
  
$
3
  
$
(234
)
 
$
165,011
 


     
Gross Unrealized
  
Estimated
 
December 31, 2017
 
Cost
  
Gains
  
Losses
  
Fair Value
 
Available-for-sale securities:
            
Corporate debt securities
 
$
132,434
  
$
—
  
$
(206
)
 
$
132,228
 
Debt securities issued by U.S. government agencies
  
38,135
   
—
   
(59
)
  
38,076
 
Total securities with a maturity of one year or less
  
170,569
   
—
   
(265
)
  
170,304
 
                 
Corporate debt securities
  
8,267
   
—
   
(35
)
  
8,232
 
Debt securities issued by U.S. government agencies
  
23,264
   
—
   
(37
)
  
23,227
 
Total securities with a maturity of one to two years
  
31,531
   
—
   
(72
)
  
31,459
 
Total available-for-sale securities
 
$
202,100
  
$
—
  
$
(337
)
 
$
201,763
 
Assets Measured at Fair Value on a Recurring Basis
The following tables present the major security types we held at June 30, 2018 and December 31, 2017 that are regularly measured and carried at fair value. The table segregates each security by the level within the fair value hierarchy of the valuation techniques we utilized to determine the respective securities' fair value (in thousands):

 
 
At
June 30, 2018
  
Quoted Prices
in Active Markets
(Level 1)
  
Significant Other
Observable Inputs
(Level 2)
 
Money market funds (1)
 
$
193,752
  
$
193,752
  
$
—
 
Corporate debt securities (2)
  
89,211
   
—
   
89,211
 
Debt securities issued by U.S. government agencies (3)
  
81,798
   
—
   
81,798
 
Total
 
$
364,761
  
$
193,752
  
$
171,009
 

 
 
At
December 31, 2017
  
Quoted Prices
in Active Markets
(Level 1)
  
Significant Other
Observable Inputs
(Level 2)
 
Money market funds (1)
 
$
48,430
  
$
48,430
  
$
—
 
Corporate debt securities (3)
  
140,460
   
—
   
140,460
 
Debt securities issued by U.S. government agencies (3)
  
61,303
   
—
   
61,303
 
Total
 
$
250,193
  
$
48,430
  
$
201,763
 
  
(1)
Included in cash and cash equivalents on our condensed consolidated balance sheets.
(2)
At June 30, 2018, $6.0 million was included in cash and cash equivalents on our condensed consolidated balance sheet, with the difference included in short-term investments on our condensed consolidated balance sheet.
(3)
Included in short-term investments on our condensed consolidated balance sheets.