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Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Billions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Disaggregation of Revenue [Line Items] |  |  |  |
Sales discounts for products | $ 2.6 | $ 2.9 | $ 7.7 | $ 8.2
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 |
Maximum |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Return period prior to expiration date of product |  |  | 6 months |
United States | Maximum |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Payment terms |  |  | 90 days |
Outside of the United States | Minimum |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Payment terms |  |  | 30 days |
Outside of the United States | Maximum |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Payment terms |  |  | 90 days |
Pharmaceutical segment | United States | Minimum |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Payment terms |  |  | 36 days |
Animal Health segment | United States | Minimum |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Payment terms |  |  | 30 days |

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