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
	Jan. 31, 2019	Feb. 29, 2012 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 KRW (₩)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018 KRW (₩)	Jun. 05, 2018 USD ($) shares	Mar. 31, 2016	Dec. 31, 2012 USD ($)	Feb. 29, 2012 KRW (₩)
Collaborative and Other Relationships (Textual)
Research and development																			$ 2,597.2	$ 2,253.6	$ 1,973.3
Product Revenues			$ 3,526.3		$ 3,439.0	$ 3,356.5	$ 3,131.1	$ 3,307.0	$ 3,077.8	$ 3,078.4	$ 2,810.7								13,452.9	12,273.9	11,448.8
Cost of Sales																			1,816.3	1,630.0	1,478.7
Collaboration profit (loss) sharing																			185.0	112.3	10.2
Potential future milestone payments commitment, approximately			5,000.0															$ 5,000.0	5,000.0
Prepaid research and development			1,690.6					431.6										1,690.6	1,690.6	431.6
Payments to Acquire Equity Method Investments																			676.6	0.0	0.0
Equity in loss of investee, net of tax																			0.0	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	800.0
Research and development						$ 324.1																$ 10.0
Estimated additional payments upon achievement of development and commercial milestones																			260.0
License Fee																			70.0
Expected License Fee And Regulatory Milestone Payments			130.0															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
Additional milestone payment																										$ 10.0
Term of collaboration agreement						10 years											6 years
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																			116.0	74.3	50.5
Research and development												$ 50.0
BAN2401 and Elenbecestat R&D expense incurred by collaboration																			232.0	146.2	95.1
Additional milestone payment			625.0															$ 625.0	625.0
Percentage of future development costs related to Eisai																		15.00%
Alkermes
Collaborative and Other Relationships (Textual)
Research and development								50.0											68.7	80.3
Upfront And Milestone Payments Made To Collaborative Partner								28.0
Potential future milestone payments commitment, approximately			150.0															$ 150.0	150.0
Bristol-Myers Squibb
Collaborative and Other Relationships (Textual)
Research and development										300.0									97.0
Additional milestone payment			410.0															410.0	410.0
iPerian
Collaborative and Other Relationships (Textual)
Additional milestone payment			490.0															490.0	490.0
Developmental Milestone Payment										$ 60.0
Acorda
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																			36.5	34.0	31.5
Expected additional milestone payments when certain sales threshold is met			15.0															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%
AGTC
Collaborative and Other Relationships (Textual)
Research and development																			$ 39.5	27.5	26.5
License Fee															$ 35.6
Upfront And Milestone Payments Made To Collaborative Partner															124.0
Prepaid research and discovery services															58.4
Purchase of common stock															30.0
Premium on purchase of common stock															7.5
Investment in common stock, value															$ 22.5
University of Pennsylvania
Collaborative and Other Relationships (Textual)
Research and development														$ 20.0					64.0	33.0	27.8
Prepaid research and development								$ 29.1						$ 15.0						29.1
Future research and development commitment			137.0															137.0	137.0
C4 Therapeutics
Collaborative and Other Relationships (Textual)
Research and development			17.0
Upfront payment for collaboration agreement			45.0															45.0	45.0
Additional milestone payment			370.0															370.0	370.0
Prepaid research and development			$ 28.0															$ 28.0	28.0
Other research and discovery
Collaborative and Other Relationships (Textual)
Research and development																			5.0	10.0	10.3
Samsung Bioepis
Collaborative and Other Relationships (Textual)
Research and development																$ 46.0
Collaboration profit (loss) sharing																			$ 187.4	$ 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%	30.00%				30.00%
Until First GAZYVA Threshold Date			39.00%					39.00%				39.00%						39.00%	39.00%	39.00%	39.00%		39.00%		39.00%
Until GAZYVA First Non-CLL FDA Approval			40.00%															40.00%	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%
GAZYVA
Collaborative and Other Relationships (Textual)
Percentage of Co promotion Operating Profits first fifty million			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	$ 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%				39.00%
After First Threshold Date and until Second Threshold Date			37.50%															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)
Research and development							$ 12.8
Product Revenues																			$ 1.4	$ 52.7	$ 7.8
Unrecorded tax benefit																				93.8
ZINBRYTA | AbbVie
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																			16.2	19.9	24.3
SPINRAZA
Collaborative and Other Relationships (Textual)
Product Revenues																			1,724.2	883.7	4.6
SPINRAZA | Ionis Pharmaceuticals
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																			285.0	234.5	257.8
Research and development																					35.3
Cost of Sales																			$ 238.0	112.4	0.5
License Fee													$ 75.0
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																			$ 53.0	12.0	5.5
Estimated additional payments upon achievement of development and commercial milestones																			55.0
Upfront And Milestone Payments Made To Collaborative Partner			$ 35.0
Biosimilars
Collaborative and Other Relationships (Textual)
Product Revenues																			545.1	379.8	100.7
IMRALDI
Collaborative and Other Relationships (Textual)
Product Revenues																			16.7	0.0	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%
Equity Method Investments		$ 45.0	$ 680.6															$ 680.6	$ 680.6				₩ 759.5				₩ 49.5
Equity Method Investment, Ownership Percentage		15.00%	49.90%															49.90%	49.90%				49.90%				15.00%
Payments to Acquire Equity Method Investments			$ 676.6	₩ 759.5
Equity Method Investment, Ownership Percentage Before Additional Purchase Transaction			5.00%															5.00%	5.00%				5.00%
Samsung Bioepis | Collaborative Arrangement
Collaborative and Other Relationships (Textual)
Product Revenues																			$ 96.4	42.7	20.2
Research and development | AbbVie
Collaborative and Other Relationships (Textual)
BAN2401 and Elenbecestat R&D expense incurred by collaboration																			$ 32.4	39.9	48.6
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 | Eisai
Collaborative and Other Relationships (Textual)
Biogen share of co-promotion profits or losses																			55.00%
U.S | AbbVie
Collaborative and Other Relationships (Textual)
Product Revenues																			$ (8.6)	(16.9)	(21.9)
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																			854.0	657.0	4.6
U.S | SPINRAZA | Ionis Pharmaceuticals
Collaborative and Other Relationships (Textual)
Product Revenues																			1,724.2	883.7	4.6
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
Foreign | AbbVie
Collaborative and Other Relationships (Textual)
Product Revenues																			1.4	52.7	7.8
Abbvie collaboration profit (loss) sharing																			$ 2.4	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
Collaborative and Other Relationships (Textual)
Net change in acquired and in-licensed rights and patents																				150.0
Percentage of development expenses to be reimbursed | Eisai
Collaborative and Other Relationships (Textual)
Percentage of future development costs related to Eisai	45.00%
AbbVie | Collaborative Arrangement
Collaborative and Other Relationships (Textual)
Product Revenues																			$ (8.6)	(16.9)	(21.9)
AbbVie | U.S | Collaborative Arrangement
Collaborative and Other Relationships (Textual)
Product Revenues																			$ (8.6)	$ (16.9)	$ (21.9)