Collaborative and Other Relationships (Details Textual) $ in Millions, ₩ in Billions	1 Months Ended		3 Months Ended												9 Months Ended	12 Months Ended
	Jan. 31, 2019	Feb. 29, 2012 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Mar. 31, 2012 USD ($)	Dec. 31, 2018	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2012 USD ($)	Feb. 29, 2012 KRW (₩)
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN			$ 2,712.4	$ 2,622.5	$ 2,639.7	$ 2,380.1	$ 2,502.9	$ 2,539.6	$ 2,466.0	$ 2,309.4						$ 10,354.7	$ 9,817.9	$ 9,188.5
Research and development																2,253.6	1,973.3	2,012.8
Other revenues			179.6	$ 48.8	41.6	$ 90.0	50.9	98.6	79.0	$ 87.9						360.0	316.4	236.1
Collaboration profit (loss) sharing																112.3	10.2	0.0
Potential future milestone payments commitment, approximately			4,200.0													4,200.0
Prepaid research and development			1,027.5				$ 1,335.8									1,027.5	1,335.8
Equity in loss of investee, net of tax																0.0	0.0	(12.5)
Roche Group - Genentech
Collaborative and Other Relationships (Textual)
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses																1,316.4	1,249.5	1,269.8
Revenue on sales in the rest of world for RITUXAN																242.8	65.0	69.4
Collaboration Expenses in Dispute																120.0
AbbVie
Collaborative and Other Relationships (Textual)
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses																$ 16.9	21.9
Biogen share of co-promotion profits or losses																50.00%
Acorda
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																$ 34.0	31.5	30.6
Expected additional milestone payments when certain sales threshold is met			15.0													15.0
Foreign sales required to trigger milestone																100.0
Ionis Pharmaceuticals, Inc.
Collaborative and Other Relationships (Textual)
Expected additional milestone payments when certain sales threshold is met			800.0													800.0
Research and development													$ 75.0			12.0		10.0
Estimated additional payments upon achievement of development and commercial milestones.			260.0													260.0
License Fee																70.0
Expected License Fee And Regulatory Milestone Payments			130.0													130.0
Upfront And Milestone Payments Made To Collaborative Partner			25.0										$ 100.0			25.0			$ 30.0
Additional milestone payment																			$ 10.0
Term of collaboration agreement													6 years
Prepaid research and discovery services													$ 25.0
Additional milestone payments for product candidate using a different modality																90.0
Eisai
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																74.3	50.5	40.4
Research and development																263.4	50.0
Total expense incurred by collaboration																146.2	95.1	84.1
Additional milestone payment			625.0													625.0
Bristol-Myers Squibb [Member]
Collaborative and Other Relationships (Textual)
Research and development					300.0
Additional milestone payment			410.0													410.0
iPerian
Collaborative and Other Relationships (Textual)
Additional milestone payment			490.0													490.0
Developmental Milestone Payment					$ 60.0
Alkermes
Collaborative and Other Relationships (Textual)
Research and development			50.0													80.3
Upfront And Milestone Payments Made To Collaborative Partner			28.0													28.0
Potential future milestone payments commitment, approximately			150.0													150.0
AGTC
Collaborative and Other Relationships (Textual)
Research and development																27.5	26.5	54.5
Upfront And Milestone Payments Made To Collaborative Partner											$ 124.0
Additional milestone payment			1,100.0													1,100.0
Prepaid research and discovery services			11.1								58.4					11.1
Purchase of common stock											30.0
Total licensing and other fees											$ 35.6
University of Pennsylvania
Collaborative and Other Relationships (Textual)
Research and development									20.0							33.0	27.8
Prepaid research and discovery services			12.7													12.7
Potential future milestone payments commitment, approximately			2,000.0													2,000.0
Prepaid research and development			29.1						$ 15.0							29.1
Future research and development commitment			$ 0.0													0.0
Other research and discovery
Collaborative and Other Relationships (Textual)
Research and development																10.0	10.3	$ 9.7
Samsung Biosimilar Agreement
Collaborative and Other Relationships (Textual)
Research and development												$ 46.0
Collaboration profit (loss) sharing																$ 111.0	$ 15.1
Equity Method Investments, Expected Profit Share																50.00%
RITUXAN
Collaborative and Other Relationships (Textual)
Percentage of Co promotion Operating Profits first fifty million			30.00%													30.00%
Until First GAZYVA Threshold Date			39.00%				39.00%									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%
GAZYVA
Collaborative and Other Relationships (Textual)
Percentage of Co promotion Operating Profits first fifty million			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%
After First Threshold Date and until Second Threshold Date			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													$ 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)
Revenue on sales in the rest of world for RITUXAN																$ 52.7	$ 7.8	$ 0.0
Net impairment charges																190.8
Unrecorded tax benefit																(93.8)
ZINBRYTA | AbbVie
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																19.9	24.3	60.8
SPINRAZA
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN																883.7	4.6	0.0
SPINRAZA | Ionis Pharmaceuticals, Inc.
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																234.5	257.8	74.9
Research and development														$ 29.0			35.3	42.8
Royalty cost of sales																$ 112.4	0.5
License Fee								$ 75.0
SPINRAZA | Ionis Pharmaceuticals, Inc. | Minimum
Collaborative and Other Relationships (Textual)
Percentage Of Royalties As Per Collaboration																11.00%
SPINRAZA | Ionis Pharmaceuticals, Inc. | Maximum
Collaborative and Other Relationships (Textual)
Percentage Of Royalties As Per Collaboration																15.00%
SOD1 | Ionis Pharmaceuticals, Inc.
Collaborative and Other Relationships (Textual)
Research and development																	5.5	20.0
Lead programs | AGTC
Collaborative and Other Relationships (Textual)
Additional milestone payment			467.5													$ 467.5
Discovery programs | AGTC
Collaborative and Other Relationships (Textual)
Additional milestone payment			592.5													592.5
Samsung Biosimilar Agreement
Collaborative and Other Relationships (Textual)
Other revenues																42.7	20.2	62.9
Additional milestone payment			$ 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%
Equity Method Investments		$ 45.0																		₩ 49.5
Percentage of equity interest to the portion of total capital stock		15.00%	5.00%													5.00%				15.00%
Equity Method Investment Ownership Percentage Maximum		49.90%
Research and development | AbbVie
Collaborative and Other Relationships (Textual)
Total expense incurred by collaboration																$ 39.9	48.6	113.8
Selling, general and administrative | ZINBRYTA | AbbVie
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																	22.0
U.S
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN																7,017.1	7,050.4	6,545.8
Other revenues																249.5	224.7	$ 142.0
U.S | AbbVie
Collaborative and Other Relationships (Textual)
Other revenues																$ 16.9	21.9
U.S | Eisai
Collaborative and Other Relationships (Textual)
Biogen share of co-promotion profits or losses																55.00%
U.S | SPINRAZA | Ionis Pharmaceuticals, Inc.
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN																$ 883.7	4.6
Outside the U.S | AbbVie
Collaborative and Other Relationships (Textual)
Collaboration profit (loss) sharing																$ 1.3	4.9
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%
Acquired and in-licensed rights and patents | Ionis Pharmaceuticals, Inc.
Collaborative and Other Relationships (Textual)
Net change in acquired and in-licensed rights and patents																$ 150.0
Acquired and in-licensed rights and patents | SPINRAZA
Collaborative and Other Relationships (Textual)
Net change in acquired and in-licensed rights and patents																90.0
Acquired and in-licensed rights and patents | Biosimilars
Collaborative and Other Relationships (Textual)
Net change in acquired and in-licensed rights and patents																$ 25.0	$ 50.0
Percentage of development expenses to be reimbursed | Eisai
Collaborative and Other Relationships (Textual)
Percentage of future development costs related to Eisai	45.00%														15.00%