Acquisitions, Collaborations and Other Arrangements (Details) $ / shares in Units, shares in Millions, $ in Millions							1 Months Ended		3 Months Ended			9 Months Ended
	Oct. 23, 2020 USD ($)	Sep. 24, 2020 USD ($) $ / shares	Aug. 25, 2020 USD ($)	Aug. 17, 2020 USD ($) program	Jul. 13, 2020 USD ($) shares	Apr. 07, 2020 USD ($) $ / shares	Oct. 31, 2020 USD ($)	Aug. 31, 2019 USD ($) shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Oct. 31, 2020 € / shares	Jul. 17, 2020 agreement	Jul. 13, 2020 € / shares	Jun. 19, 2020 agreement	May 29, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Aug. 31, 2019 € / shares	Jul. 31, 2018 USD ($)
Acquisition, Collaborations and Other Arrangements [Line Items]
Acquired in-process research and development expenses									$ 1,171		$ 3,960	$ 5,792	$ 4,251
Stock-based compensation expense												482	479
Payments to acquire in process research and development												5,804	4,251
Prepaid and other current assets									2,030			2,030							$ 1,440
Net unrealized loss on investment of equity securities									964			1,046
Arcus
Acquisition, Collaborations and Other Arrangements [Line Items]
Equity securities owned (in shares) | shares																		2.2
Equity securities in offering																		$ 61
Pionyr
Acquisition, Collaborations and Other Arrangements [Line Items]
Number of agreements | agreement																	2
Equity method investment, ownership percentage																	49.90%
Tizona
Acquisition, Collaborations and Other Arrangements [Line Items]
Number of agreements | agreement															2
Equity method investment, ownership percentage															49.90%
the “Collaboration Agreement" and the “Stock Purchase Agreements” | Arcus
Acquisition, Collaborations and Other Arrangements [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners					13.00%
the “Collaboration Agreement" and the “Stock Purchase Agreements” | Arcus
Acquisition, Collaborations and Other Arrangements [Line Items]
Share price (usd per share) | € / shares																€ 22.67
Acquired in-process research and development expenses					$ 256				100
Equity securities owned (in shares) | shares					8.2
Equity securities in offering					$ 200
Payments to acquire in process research and development					175
Total upfront payments made					$ 391
Additional equity securities owned (in shares) | shares					6.0
Equity investment									135			135
Issuance discount (premium)					$ (65)
Payments for direct transactional costs					16
Additional option fee on second anniversary					100
Additional option fee on 4th, 6th, and 8th anniversaries					$ 100
Collaborative term					10 years
the “Collaboration Agreement" | Arcus
Acquisition, Collaborations and Other Arrangements [Line Items]
Future maximum milestone payments					$ 1,200
the “Stock Purchase Agreements” | Arcus
Acquisition, Collaborations and Other Arrangements [Line Items]
Purchase period					5 years
Maximum percentage of outstanding stock allowed to be purchased					35.00%
Restriction period					3 years
the "Pionyr Merger and Option Agreements" | Pionyr
Acquisition, Collaborations and Other Arrangements [Line Items]
Acquired in-process research and development expenses									215
Payments to acquire equity method investments					$ 269
Additional payable accrued					6
Estimated fair value to acquire equity method investment					70
Option exercise fee					315
Potential future maximum milestone payments					1,200
Research and Development Service Agreement | Pionyr
Acquisition, Collaborations and Other Arrangements [Line Items]
Acquired in-process research and development expenses									80
Potential maximum future milestone payments					$ 115
the "Tizona Merger and Option Agreements" | Tizona
Acquisition, Collaborations and Other Arrangements [Line Items]
Acquired in-process research and development expenses									272
Payments to acquire equity method investments			$ 302
Estimated fair value to acquire equity method investment			41
Option fee and potential future milestone payments			$ 1,300
Development Agreement | Tizona
Acquisition, Collaborations and Other Arrangements [Line Items]
Acquired in-process research and development expenses									115
the “Collaboration and Stock Purchase Agreements” | Tango
Acquisition, Collaborations and Other Arrangements [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners				7.00%
the “Collaboration and Stock Purchase Agreements” | Tango
Acquisition, Collaborations and Other Arrangements [Line Items]
Acquired in-process research and development expenses									125
Payments to acquire in process research and development				$ 125
Collaborative term				7 years
Opt-in, extension and milestone payments				$ 410
Number of programs | program				15
Cash payments made related to equity investments				$ 20
the "Collaboration Agreement" and "Subscription Agreement" | Galapagos
Acquisition, Collaborations and Other Arrangements [Line Items]
Share price (usd per share) | € / shares																				€ 140.59
Acquired in-process research and development expenses								$ 3,900
Equity securities owned (in shares) | shares								13.6
Additional equity securities owned (in shares) | shares								6.8
Issuance discount (premium)								$ 63
License and option rights paid upon closing								$ 5,100
Equity method investment, ownership percentage								22.00%
Equity method investment								$ 1,100
Prepaid and other current assets									505			505
Net unrealized loss on investment of equity securities									923
Gadeta Collaboration Arrangement
Acquisition, Collaborations and Other Arrangements [Line Items]
Noncontrolling interest in VIE									82			82									$ 82
Other Arrangements
Acquisition, Collaborations and Other Arrangements [Line Items]
Upfront collaboration expenses related to other collaborative arrangements									7		40	129	331
Cash payments made related to equity investments									$ 36		$ 15	$ 61	$ 119
Subsequent Event | the "License and Stock Purchase Agreement" | Jounce
Acquisition, Collaborations and Other Arrangements [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners							14.00%
Subsequent Event | the "License and Stock Purchase Agreement" | Jounce
Acquisition, Collaborations and Other Arrangements [Line Items]
Share price (usd per share) | € / shares														€ 10.06
Total upfront payments made							$ 120
Potential maximum future milestone payments							685
Cash payments made related to equity investments							$ 56
Subsequent Event | Three-Year Senior Term Loan Facility | Medium-term Notes
Acquisition, Collaborations and Other Arrangements [Line Items]
Borrowing from senior unsecured term loan facility	$ 1,000
Immunomedics
Acquisition, Collaborations and Other Arrangements [Line Items]
Acquisition consideration transferred		$ 21,000
Acquisition share price (usd per share) | $ / shares		$ 88.00
Forty Seven
Acquisition, Collaborations and Other Arrangements [Line Items]
Share price (usd per share) | $ / shares						$ 95.50
Total consideration, net of acquired cash						$ 4,700
Acquired in-process research and development expenses						4,500
Assets acquired, net of cash						$ 202
Stock-based compensation expense										$ 144