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Concentrations of Credit and Other Risks (Tables)
 12 Months Ended
Dec. 31, 2015
Concentrations of Credit and Other Risks [Abstract]  
Product Revenue by Geographic Destination as a Percentage of Total Product Revenues
Product revenue by geographic destination as a percentage of total product revenues were as follows for the years ended December 31:

  
2015
  
2014
  
2013
 
       
Domestic
  
79
%
  
82
%
  
85
%
Foreign
  
21
   
18
   
15