Collaborative and Other Relationships (Details Textual) $ in Millions, ₩ in Billions		1 Months Ended	3 Months Ended														6 Months Ended	12 Months Ended
		Feb. 29, 2012 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Dec. 31, 2012 USD ($)	Jun. 30, 2012 USD ($)	Mar. 31, 2012 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)	Feb. 29, 2012 KRW (₩)	Jan. 31, 2007
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN			$ 2,502.9	$ 2,539.6	$ 2,466.0	$ 2,309.4	$ 2,425.9	$ 2,391.7	$ 2,198.6	$ 2,172.3								$ 9,817.9	$ 9,188.5	$ 8,203.4
Research and development																		1,973.3	2,012.8	1,893.4
Other revenues from external customers			50.9	$ 98.6	79.0	$ 87.9	79.5	49.0	$ 55.6	$ 52.0								316.4	236.1	304.5
Prepaid research and development			1,335.8				1,107.6											1,335.8	1,107.6
Potential future milestone payments commitment, approximately			3,100.0															3,100.0
Income (Loss) from Equity Method Investments																		0.0	(12.5)	(15.1)
Biogen Hemophilia
Collaborative and Other Relationships (Textual)
Stock acquired in acquisition																							100.00%
Acorda
Collaborative and Other Relationships (Textual)
Expected additional milestone payments for successful achievement of regulatory and commercial sales milestones			375.0															375.0
Expected additional milestone payments when certain sales threshold is met			15.0															15.0
Foreign sales required to trigger milestone																		100.0
Biogen's share of expense reflected within our consolidation statements of income																		31.5	30.6	29.2
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																		21.9
Total expense incurred by collaboration																		$ 50.0
Biogen share of co-promotion profits or losses																		50.00%
Swedish Orphan Biovitrum
Collaborative and Other Relationships (Textual)
Consideration per product																		$ 10.0
Reimbursement cost achieving period																		6 years
Time period for paying remaining balance due																		90 days
Ionis Pharmaceuticals, Inc.
Collaborative and Other Relationships (Textual)
Research and development													$ 75.0	$ 30.0				$ 5.5	20.0	20.0
Term of collaboration agreement													6 years
Total upfront and milestone payments made to collaborative partner													$ 100.0
Prepaid research and discovery services													$ 25.0
License Fee																		70.0
Expected License Fee And Regulatory Milestone Payments			130.0															130.0
Additional milestone payments for product candidate using a different modality																		90.0
Additional milestone payment			10.0															10.0
Eisai
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																		50.5	40.4	29.1
Research and development											$ 21.6							50.0
Total expense incurred by collaboration																		95.1	84.1	57.5
Total upfront and milestone payments made to collaborative partner																					$ 100.0
Additional milestone payment			1,000.0															1,000.0
Milestone payment made during period											$ 35.0
AGTC
Collaborative and Other Relationships (Textual)
Research and development								48.1										26.5	54.5
Total upfront and milestone payments made to collaborative partner								124.0
Prepaid research and discovery services								58.4
Additional milestone payment			1,100.0															1,100.0
Purchase of common stock								30.0
Total licensing and other fees								$ 35.6
Research and development services																			6.5
Premium on equity investment																			7.5
University of Pennsylvania
Collaborative and Other Relationships (Textual)
Research and development																	$ 20.0
Prepaid research and development					$ 15.0												$ 15.0
Future research and development commitment			47.5															47.5
Potential future milestone payments commitment, approximately			2,000.0															2,000.0
Sangamo BioSciences
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																		13.4	13.6	28.9
Total upfront and milestone payments made to collaborative partner																					20.0
Additional milestone payment			$ 300.0															300.0
Mitsubishi Tanabe Pharma Corporation
Collaborative and Other Relationships (Textual)
Research and development							$ 60.0											22.8
Other research and discovery
Collaborative and Other Relationships (Textual)
Research and development																		10.3	$ 9.7	$ 40.0
Samsung Biosimilar Agreement
Collaborative and Other Relationships (Textual)
Research and development												$ 46.0
Collaboration profit (loss) sharing																		$ 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%
Percentage of share of Co promotion profits exceeding $50 million			39.00%				40.00%											39.00%	40.00%	40.00%
Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option One			39.00%															39.00%
Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option One			40.00%															40.00%
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
Percentage Of Co Promotion Operating Losses																		35.00%
Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option One			39.00%															39.00%
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																		38.00%
Ocrelizumab
Collaborative and Other Relationships (Textual)
Reduction in royalty rate																		50.00%
Period of collaboration agreement																		11 years
Ocrelizumab | Minimum
Collaborative and Other Relationships (Textual)
Future royalties percentage to be received on sale of ocrelizumab																		13.50%
Ocrelizumab | 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																		$ 7.8	$ 0.0	$ 0.0
Research and development																			16.0
ZINBRYTA | AbbVie
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																		24.3	60.8	67.4
ELOCTATE
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN																		513.2	319.7	58.4
ELOCTATE | Swedish Orphan Biovitrum
Collaborative and Other Relationships (Textual)
Research and development																		144.0
ALPROLIX | Swedish Orphan Biovitrum
Collaborative and Other Relationships (Textual)
Research and development																		123.0
SPINRAZA
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN																		4.6	0.0	0.0
SPINRAZA | Ionis Pharmaceuticals, Inc.
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																		257.8	74.9	27.7
Research and development																$ 29.0		35.3	42.8	57.3
Estimated additional payments upon achievement of development and commercial milestones.			$ 90.0															90.0
SOD1 | Ionis Pharmaceuticals, Inc.
Collaborative and Other Relationships (Textual)
Research and development																			10.0
DMPK | Ionis Pharmaceuticals, Inc.
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income																		3.1	9.0	10.9
Research and development															$ 12.0
Additional milestone payment							$ 4.2								$ 59.0				4.2
Lead programs | AGTC
Collaborative and Other Relationships (Textual)
Research and development																			5.0
Additional milestone payment			472.5															472.5
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 from external customers																		20.2	62.9	58.5
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%	6.50%															6.50%				15.00%
Equity Method Investment Ownership Percentage Maximum		49.90%
Revenue | Swedish Orphan Biovitrum
Collaborative arrangements and non-collaborative arrangement transactions
Biogen effective royalty rate																		0.14
Cost of sales | Swedish Orphan Biovitrum
Collaborative arrangements and non-collaborative arrangement transactions
Biogen effective royalty rate																		0.11
Research and development | AbbVie
Collaborative and Other Relationships (Textual)
Total expense incurred by collaboration																		$ 48.6	113.8	117.8
Research and development | Eisai
Collaborative and Other Relationships (Textual)
Amount recorded to reflect fair value of options granted																					$ 17.7
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																		7,050.4	6,545.8	5,566.7
Other revenues from external customers																		224.7	142.0	212.6
U.S | 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																		1,249.5	1,269.8	1,117.1
U.S | AbbVie
Collaborative and Other Relationships (Textual)
Other revenues from external customers																		21.9
Outside the U.S | RITUXAN | Roche Group - Genentech
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN																		$ 65.0	69.4	78.3
Outside the U.S | Ocrelizumab
Collaborative and Other Relationships (Textual)
Future royalties percentage to be received on sale of ocrelizumab																		3.00%
Europe
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN	[1]																	$ 1,533.5	1,497.6	1,383.9
Other revenues from external customers	[1]																	70.0	$ 29.6	$ 31.6
Europe | AbbVie
Collaborative and Other Relationships (Textual)
Collaboration profit (loss) sharing																		4.9
Acquired and in-licensed rights and patents | ZINBRYTA
Collaborative and Other Relationships (Textual)
Net change in acquired and in-licensed rights and patents																		32.0
Acquired and in-licensed rights and patents | ALPROLIX
Collaborative and Other Relationships (Textual)
Net change in acquired and in-licensed rights and patents																		26.5
Milestone payment made during period																		20.0
Acquired and in-licensed rights and patents | SPINRAZA
Collaborative and Other Relationships (Textual)
Net change in acquired and in-licensed rights and patents																		60.0
Acquired and in-licensed rights and patents | Biosimilars
Collaborative and Other Relationships (Textual)
Net change in acquired and in-licensed rights and patents																		$ 50.0

[1] Represents amounts related to Europe less those attributable to Germany.