Collaborative and Other Relationships (Details Textual) shares in Millions, $ in Millions, ₩ in Billions	1 Months Ended	3 Months Ended													9 Months Ended	12 Months Ended
	Feb. 29, 2012 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 KRW (₩)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2019 KRW (₩)	Dec. 31, 2018 KRW (₩)	Nov. 07, 2018 USD ($)	Jun. 05, 2018 USD ($) shares	Sep. 30, 2017	Mar. 31, 2016	Dec. 31, 2012 USD ($)	Feb. 29, 2012 KRW (₩)
Collaborative and Other Relationships (Textual)
Research and development																$ 2,280.6	$ 2,597.2	$ 2,253.6
Product Revenues		$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 3,526.3		$ 3,439.0	$ 3,356.5	$ 3,131.1						14,377.9	13,452.9	12,273.9
Cost of Sales																1,955.4	1,816.3	1,630.0
Collaboration profit (loss) sharing																241.6	185.0	112.3
Potential future milestone payments commitment, approximately		6,800.0														6,800.0
Prepaid research and development		1,252.8				1,690.6									$ 1,690.6	1,252.8	1,690.6
Payments to increase investment in Samsung Bioepis																0.0	676.6	0.0
Equity in loss of investee, net of tax																(79.4)	0.0	0.0
Ionis Pharmaceuticals
Collaborative and Other Relationships (Textual)
Expected additional milestone payments when certain sales threshold is met		800.0														800.0
Research and development									324.1							30.0			$ 10.0
Estimated additional payments upon achievement of development and commercial milestones																260.0
Net change in acquired and in-licensed rights and patents																		150.0
License Fee																70.0
Expected License Fee And Regulatory Milestone Payments		130.0														130.0
Upfront payment for collaboration agreement																							$ 375.0			$ 30.0
Investment in common stock, shares purchased | shares																							11.5
Upfront And Milestone Payments Made To Collaborative Partner											$ 25.0
Research and development expense asset acquired		45.0							$ 486.2
Additional milestone payment		155.0														155.0										$ 10.0
Term of collaboration agreement									10					six
Total payment to enter collaboration agreement																							$ 1,000.0
Prepaid research and discovery services									$ 50.9
Purchase of common stock																							625.0
Premium on purchase of common stock																							162.1
Investment in common stock, value																							$ 462.9
Ionis Pharmaceuticals | Minimum
Collaborative and Other Relationships (Textual)
Estimated additional payments upon achievement of development and commercial milestones																125.0
Ionis Pharmaceuticals | Maximum
Collaborative and Other Relationships (Textual)
Estimated additional payments upon achievement of development and commercial milestones																270.0
Eisai
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																174.3	116.0	74.3
BAN2401 and Elenbecestat R&D expense incurred by collaboration																$ 348.7	232.0	146.2
Percentage of future development costs related to Eisai															15.00%	45.00%
Loss on Contract Termination			$ 48.0		45.0
Alkermes
Collaborative and Other Relationships (Textual)
Research and development											50.0					$ 53.5	68.7	80.3
Upfront And Milestone Payments Made To Collaborative Partner											$ 28.0
Potential future milestone payments commitment, approximately		155.0														155.0
Bristol-Myers Squibb
Collaborative and Other Relationships (Textual)
Research and development												$ 300.0				144.0	97.0
Additional milestone payment		360.0														360.0
iPerian
Collaborative and Other Relationships (Textual)
Additional milestone payment		370.0														370.0
Developmental Milestone Payment												$ 60.0
Acorda
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																42.0	36.5	34.0
Expected additional milestone payments when certain sales threshold is met		15.0														15.0
Foreign sales required to trigger milestone																$ 100.0
AbbVie
Collaborative and Other Relationships (Textual)
Biogen share of co-promotion profits or losses																50.00%
Skyhawk Therapeutics
Collaborative and Other Relationships (Textual)
Upfront And Milestone Payments Made To Collaborative Partner		15.0			74.0
Research and development expense asset acquired		8.0			38.5
Additional milestone payment		2,400.0														$ 2,400.0
Prepaid research and discovery services		7.0			$ 35.5											7.0
Other research and discovery
Collaborative and Other Relationships (Textual)
Research and development																30.6	22.0	10.0
Samsung Bioepis
Collaborative and Other Relationships (Textual)
Research and development													$ 46.0
Estimated additional payments upon achievement of development and commercial milestones																210.0
Contract Option Exercise Fee																60.0
Product Revenues																106.2	96.4	42.7
Collaboration profit (loss) sharing																241.6	187.4	$ 111.0
Upfront And Milestone Payments Made To Collaborative Partner		100.0
Research and development expense asset acquired		63.0
Prepaid research and discovery services		37.0														$ 37.0
Equity Method Investments, Expected Profit Share																50.00%
Due from Related Parties		85.0				116.9									$ 116.9	$ 85.0	116.9
Due to Related Parties		$ 100.0				$ 31.5									$ 31.5	$ 100.0	$ 31.5
RITUXAN
Collaborative and Other Relationships (Textual)
Percentage of Co promotion Operating Profits first fifty million		30.00%														30.00%				30.00%
Until First GAZYVA Threshold Date		39.00%									37.50%					39.00%		37.50%		39.00%					39.00%
Until GAZYVA First Non-CLL FDA Approval		40.00%														40.00%				40.00%
After First Threshold Date and until Second Threshold Date		37.50%				37.50%									37.50%	37.50%	37.50%			37.50%	37.50%			37.50%
GAZYVA
Collaborative and Other Relationships (Textual)
Percentage of Co promotion Operating Profits first fifty million		35.00%									35.00%					35.00%		35.00%		35.00%
Co-promotion operating profit threshold for Rituxan in U S and Canada to determine share of co promotion operating profit prior to amendment		$ 50.0														$ 50.0
Limit of gross sale of GAZYVA to be achieved in preceding 12 months under option one																150.0
Limit of gross sale of GAZYVA to be achieved in any 12 months under option one																150.0
Sales Trigger Gross Sales Threshold																$ 500.0
Until First GAZYVA Threshold Date		39.00%														39.00%				39.00%
After First Threshold Date and until Second Threshold Date		37.50%														37.50%				37.50%
New Anti-CD20 | Minimum
Collaborative and Other Relationships (Textual)
Future percentage of co-promotion operating profits																30.00%
New Anti-CD20 | Maximum
Collaborative and Other Relationships (Textual)
Future percentage of co-promotion operating profits																37.50%
OCREVUS
Collaborative and Other Relationships (Textual)
Royalty operating profit threshold for highest royalty rate percentage																$ 900.0
Reduction in royalty rate																50.00%
Period of collaboration agreement																11 years
OCREVUS | Minimum
Collaborative and Other Relationships (Textual)
Future royalties percentage to be received on sale of ocrelizumab																13.50%
OCREVUS | Maximum
Collaborative and Other Relationships (Textual)
Future royalties percentage to be received on sale of ocrelizumab																24.00%
ZINBRYTA
Collaborative and Other Relationships (Textual)
Product Revenues																$ 0.0	$ 1.4	$ 52.7
Unrecorded tax benefit																		93.8
ZINBRYTA | AbbVie
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																0.3	16.2	19.9
SPINRAZA
Collaborative and Other Relationships (Textual)
Product Revenues																2,097.0	1,724.2	883.7
SPINRAZA | Ionis Pharmaceuticals
Collaborative and Other Relationships (Textual)
Product Revenues																2,097.0	1,724.2	883.7
Cost of Sales																$ 293.0	238.0	112.4
SPINRAZA | Ionis Pharmaceuticals | Minimum
Collaborative and Other Relationships (Textual)
Percentage Of Royalties As Per Collaboration																11.00%
SPINRAZA | Ionis Pharmaceuticals | Maximum
Collaborative and Other Relationships (Textual)
Percentage Of Royalties As Per Collaboration																15.00%
SOD1 | Ionis Pharmaceuticals
Collaborative and Other Relationships (Textual)
Research and development																$ 20.0	18.0	12.0
Estimated additional payments upon achievement of development and commercial milestones																55.0
Upfront And Milestone Payments Made To Collaborative Partner						$ 35.0
VUMERITY
Collaborative and Other Relationships (Textual)
Product Revenues																$ 5.5	0.0	0.0
Net change in acquired and in-licensed rights and patents		$ 155.0
VUMERITY | Alkermes
Collaborative and Other Relationships (Textual)
Percentage Of Royalties As Per Collaboration																15.00%
Biosimilars
Collaborative and Other Relationships (Textual)
Product Revenues																$ 738.3	545.1	379.8
IMRALDI
Collaborative and Other Relationships (Textual)
Product Revenues																184.0	16.7	0.0
Samsung Bioepis
Collaborative and Other Relationships (Textual)
Net change in acquired and in-licensed rights and patents																		75.0
Additional milestone payment						25.0									$ 25.0		25.0
Investments By Third Party In Joint Venture As Per Agreement.	$ 250.0																										₩ 280.5
Joint Venture Owner Ship Percentage By Third Party.	85.00%
Investment in Samsung Bioepis	$ 45.0	$ 580.2				680.6									$ 680.6	$ 580.2	$ 680.6			₩ 670.8	₩ 759.5						₩ 49.5
Samsung Bioepis, ownership percentage	15.00%	49.90%														49.90%				49.90%							15.00%
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity																						$ 675.0
Income (Loss) from Equity Method Investments, Net of Dividends or Distributions																$ 79.4
Payments to increase investment in Samsung Bioepis						$ 676.6	₩ 759.5
Samsung Bioepis, ownership percentage before additional purchase transaction						5.00%									5.00%		5.00%				5.00%
Equity in loss of investee, net of tax																1.2
Amortization																(78.2)
Research and development | AbbVie
Collaborative and Other Relationships (Textual)
BAN2401 and Elenbecestat R&D expense incurred by collaboration																0.6	$ 32.4	39.9
Selling, general and administrative | Aducanumab [Member] | Eisai
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																16.2	5.4	0.0
BAN2401 and Elenbecestat expense incurred by the collaboration																$ 32.4	10.7	0.0
U.S | Eisai
Collaborative and Other Relationships (Textual)
Biogen share of co-promotion profits or losses																55.00%
U.S | ZINBRYTA
Collaborative and Other Relationships (Textual)
Product Revenues																$ 0.0	0.0	0.0
U.S | SPINRAZA
Collaborative and Other Relationships (Textual)
Product Revenues																933.4	854.0	657.0
U.S | VUMERITY
Collaborative and Other Relationships (Textual)
Product Revenues																5.5	0.0	0.0
U.S | Biosimilars
Collaborative and Other Relationships (Textual)
Product Revenues																0.0	0.0	0.0
U.S | IMRALDI
Collaborative and Other Relationships (Textual)
Product Revenues																$ 0.0	0.0	0.0
Outside the U.S | OCREVUS
Collaborative and Other Relationships (Textual)
Future royalties percentage to be received on sale of ocrelizumab																3.00%
European Union | Eisai
Collaborative and Other Relationships (Textual)
Biogen share of co-promotion profits or losses																68.50%
JAPAN | Eisai
Collaborative and Other Relationships (Textual)
Biogen share of co-promotion profits or losses																20.00%
AbbVie | Collaborative Arrangement
Collaborative and Other Relationships (Textual)
Product Revenues																$ 0.0	(8.6)	(16.9)
AbbVie | U.S | Collaborative Arrangement
Collaborative and Other Relationships (Textual)
Product Revenues																	$ (8.6)	$ (16.9)
Inventories | Samsung Bioepis
Collaborative and Other Relationships (Textual)
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity																						115.0
Equity method investment basis difference amortization period																1 year 6 months
Completed technology | Samsung Bioepis
Collaborative and Other Relationships (Textual)
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity																						615.0
Equity method investment basis difference amortization period																15 years
In-process research and development | Samsung Bioepis
Collaborative and Other Relationships (Textual)
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity																						170.0
Deferred tax liability | Samsung Bioepis
Collaborative and Other Relationships (Textual)
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity																						$ 225.0
Development Milestones
Collaborative and Other Relationships (Textual)
Potential future milestone payments commitment, approximately		$ 1,200.0														$ 1,200.0
Development Milestones | Samsung Bioepis
Collaborative and Other Relationships (Textual)
Additional milestone payment		$ 15.0														$ 15.0