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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Billions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Disaggregation of Revenue [Line Items]    
Sales discounts for products $ 2.4 $ 2.5
Return period after expiration date of product 12 months  
Minimum    
Disaggregation of Revenue [Line Items]    
Return period prior to expiration date of product 3 months  
Minimum | United States    
Disaggregation of Revenue [Line Items]    
Payment terms 36 days  
Minimum | Outside of the United States    
Disaggregation of Revenue [Line Items]    
Payment terms 30 days  
Maximum    
Disaggregation of Revenue [Line Items]    
Return period prior to expiration date of product 6 months  
Maximum | United States    
Disaggregation of Revenue [Line Items]    
Payment terms 90 days  
Maximum | Outside of the United States    
Disaggregation of Revenue [Line Items]    
Payment terms 90 days