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ACQUISITIONS - Narrative (Details) € in Millions				3 Months Ended				9 Months Ended
	Mar. 04, 2021 USD ($)	Mar. 04, 2021 EUR (€)	Oct. 23, 2020 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2021 USD ($)	Mar. 04, 2021 EUR (€)
Business Combination, Separately Recognized Transactions [Line Items]
Acquired in-process research and development expenses				$ 19,000,000		$ 1,171,000,000		$ 177,000,000	$ 5,792,000,000
Forty Seven, Inc.
Business Combination, Separately Recognized Transactions [Line Items]
Consideration, net of acquired cash							$ 4,700,000,000
Acquired share-based compensation expense									144,000,000
Acquired IPR&D | Forty Seven, Inc.
Business Combination, Separately Recognized Transactions [Line Items]
Acquired in-process research and development expenses									$ 4,500,000,000
MYR GmbH
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition consideration transferred	$ 1,600,000,000	€ 1,300
Cash paid for acquisition	1,200,000,000	€ 1,000
Fair value of contingent liability	341,000,000
Finite-lived intangible assets	845,000,000
Goodwill	226,000,000
Expected tax deductible goodwill				0				0
MYR GmbH | Level 3 | Fair Value, Measurements, Recurring
Business Combination, Separately Recognized Transactions [Line Items]
Liability for MYR GmbH (“MYR”) contingent consideration				$ 328,000,000	$ 0			$ 328,000,000		$ 341,000,000
MYR GmbH | Maximum
Business Combination, Separately Recognized Transactions [Line Items]
Liability for MYR GmbH (“MYR”) contingent consideration | €											€ 300
MYR GmbH | Hepcludex For HDV
Business Combination, Separately Recognized Transactions [Line Items]
Finite-lived intangible assets	$ 845,000,000
Weighted average useful life	10 years	10 years
MYR GmbH | Hepcludex For HDV | Discount Rate
Business Combination, Separately Recognized Transactions [Line Items]
Measurement input	0.12										0.12
MYR GmbH | Acquired IPR&D | Discount Rate
Business Combination, Separately Recognized Transactions [Line Items]
Measurement input	0.12										0.12
Immunomedics
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition consideration transferred			$ 20,600,000,000
Goodwill			$ 3,991,000,000
Immunomedics | Three-Year Senior Term Loan Facility | Medium-term Notes
Business Combination, Separately Recognized Transactions [Line Items]
Borrowing from senior unsecured term loan facility					$ 1,000,000,000

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