| Schedule of Assets at Fair Value on Recurring Basis | 
 
 Assets carried at fair value are categorized into one of the three
 categories described in Note 2 – “Accounting
 Policies” and consisted of the following (in thousands): 
 
   
 
 
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 December 31, 2016 | 
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 December 31, 2015 | 
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 Fair Value | 
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 Level 1 | 
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 Level 2 | 
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 Level 3 | 
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 Fair Value | 
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 Level 1 | 
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 Level 2 | 
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 Level 3 | 
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 Assets held in rabbi trust 
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 $ | 
 7,337 | 
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 $ | 
 
 —   | 
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 $ | 
 7,337 | 
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 $ | 
 
        —   | 
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 $ | 
 5,661 | 
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 $ | 
 
 —   | 
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 $ | 
 5,661 | 
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 $ | 
 
        —   | 
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 Cash and cash equivalents: 
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 Cash 
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 $ | 
 34,881 | 
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 $ | 
 34,881 | 
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 $ | 
 
 —   | 
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 $ | 
 
 —   | 
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 $ | 
 90,198 | 
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 $ | 
 90,198 | 
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 $ | 
 
 —   | 
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 $ | 
 
 —   | 
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 Commercial Paper 
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 9,987 | 
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 —   | 
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 9,987 | 
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 —   | 
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 Money market funds 
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 142,503 | 
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 142,503 | 
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 —   | 
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 —   | 
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 5,987 | 
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 5,987 | 
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 $ | 
 187,371 | 
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 $ | 
 177,384 | 
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 $ | 
 9,987 | 
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 $ | 
 
 —   | 
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 $ | 
   96,185 | 
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 $ | 
   96,185 | 
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 $ | 
 
 —   | 
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 $ | 
 
 —   | 
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 Marketable securities, available-for-sale: 
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 Short-term investments: 
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 U.S. Treasuries 
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 $ | 
 24,957 | 
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 $ | 
 24,957 | 
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 $ | 
 
 —   | 
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 $ | 
 
 —   | 
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 $ | 
 62,272 | 
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 $ | 
 62,272 | 
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 $ | 
 
 —   | 
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 $ | 
 
 —   | 
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 U.S. Government Sponsored Entities 
  | 
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 2,497 | 
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 —   | 
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 2,497 | 
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 —   | 
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 17,559 | 
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 —   | 
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 17,559 | 
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 —   | 
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 Asset-backed securities and other 
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 29 | 
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 $ | 
 27,454 | 
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 $ | 
 24,957 | 
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 $ | 
 2,497 | 
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 $ | 
 
 —   | 
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 $ | 
 79,860 | 
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 $ | 
 62,272 | 
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 $ | 
   17,588 | 
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 $ | 
 
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 Long-term investments: 
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 U.S. Treasuries 
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 $ | 
 40,636 | 
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 $ | 
 40,636 | 
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 $ | 
 
 —   | 
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 $ | 
 
 —   | 
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 $ | 
 15,171 | 
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 $ | 
 15,171 | 
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 $ | 
 
 —   | 
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 $ | 
 
 —   | 
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 U.S. Government Sponsored Entities 
  | 
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 12,560 | 
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 —   | 
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 12,560 | 
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 —   | 
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 12,022 | 
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 —   | 
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 12,022 | 
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 —   | 
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 Corporate debt securities 
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 17,750 | 
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 —   | 
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   17,750 | 
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 —   | 
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 16,705 | 
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 —   | 
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 16,705 | 
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 —   | 
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 Asset-backed securities and other 
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 6,529 | 
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 —   | 
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 6,529 | 
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 —   | 
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 10,497 | 
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 —   | 
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 10,497 | 
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 —   | 
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 $ | 
 77,475 | 
   | 
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 $ | 
 40,636 | 
   | 
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 $ | 
 36,839 | 
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 $ | 
 
 —   | 
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 $ | 
 54,395 | 
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 $ | 
 15,171 | 
   | 
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 $ | 
 39,224 | 
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 $ | 
 
 —   | 
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