Financial Instruments - Information About Changes in Liabilities for Contingent Consideration (Details) $ in Millions	9 Months Ended
Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 767	$ 788
Additions	97	0
Changes in estimated fair value	71	52
Payments	(106)	(85)
Fair value, ending balance	829	755
Current liability			$ 140
Contingent consideration	829	$ 755	829
Sanofi Pasteur
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	625
Fair value, ending balance	637
Contingent consideration	$ 625		$ 637
Contingent consideration, measurement input, discount rate (as percent)			0.115
Measurement Input, Discount Rate \| Sanofi Pasteur
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Contingent consideration, measurement input, discount rate (as percent)			0.08

Amount of liabilities incurred by the acquirer as part of consideration transferred in a business combination.

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Amount of increase (decrease) in the value of a contingent consideration liability, including, but not limited to, differences arising upon settlement.

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Amount of liability recognized arising from contingent consideration in a business combination.

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Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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