Collaborative and Other Relationships (Details Textual) $ in Millions, ₩ in Billions	1 Months Ended	3 Months Ended													12 Months Ended
	Feb. 29, 2012 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Dec. 31, 2012 USD ($)	Jun. 30, 2012 USD ($)	Mar. 31, 2012 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2015 KRW (₩)	Dec. 31, 2014 KRW (₩)	Feb. 29, 2012 KRW (₩)	Jan. 31, 2007
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN		$ 2,425.9	$ 2,391.7	$ 2,198.6	$ 2,172.3	$ 2,287.0	$ 2,117.3	$ 2,056.3	$ 1,742.8						$ 9,188.5	$ 8,203.4	$ 5,542.3
Research and development															2,012.8	1,893.4	1,444.1
Income (Loss) from Equity Method Investments															(12.5)	(15.1)	(17.2)
Other revenues from external customers		79.5	49.0	$ 55.6	$ 52.0	$ 49.2	$ 103.4	61.9	90.1						236.1	304.5	263.9
Biogen Hemophilia
Collaborative and Other Relationships (Textual)
Stock acquired in acquisition																						100.00%
Milestone Payments Made During Period								20.0										$ 40.0
Genentech
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN																	41.2
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses																	8.5
Elan
Collaborative and Other Relationships (Textual)
Reimbursements of research and developments expense from Elan																	11.7
Reimbursements of selling general and administrative expenses from Elan																	20.6
Amount reflected in collaboration profit sharing line for collaboration																	85.4
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															30.6	29.2	24.3
Swedish Orphan Biovitrum
Collaborative and Other Relationships (Textual)
Consideration per product															$ 10.0
Percentage of reimbursement expenses															50.00%
Percentage of development reimbursement expenses															100.00%
Reimbursement cost achieving period															6 years
Time period for paying remaining balance due															90 days
Term for revocation option right															18 months
AbbVie
Collaborative and Other Relationships (Textual)
Research and development															$ 16.0
Biogen's share of expense reflected within our consolidation statements of income															60.8	67.4	71.0
Estimated additional payments upon achievement of development and commercial milestones.		32.0													32.0
Ionis Pharmaceuticals
Collaborative and Other Relationships (Textual)
Research and development											$ 75.0	$ 30.0			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)
Research and development								21.6	117.7
Biogen's share of expense reflected within our consolidation statements of income															40.4	29.1	$ 0.0
Total upfront and milestone payments made to collaborative partner									100.0
Additional Milestone Payment		1,000.0													1,000.0
Milestone Payments Made During Period								$ 35.0
Sangamo BioSciences
Collaborative and Other Relationships (Textual)
Biogen's share of expense reflected within our consolidation statements of income															13.6	28.9
Total upfront and milestone payments made to collaborative partner									20.0
Additional Milestone Payment		300.0													300.0
AGTC
Collaborative and Other Relationships (Textual)
Research and development			48.1												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
Mitsubishi Tanabe Pharma Corporation
Collaborative and Other Relationships (Textual)
Research and development		60.0
Additional Milestone Payment		$ 484.0													484.0
Other research and discovery
Collaborative and Other Relationships (Textual)
Research and development															$ 9.7	$ 40.0
Samsung Biosimilar Agreement
Collaborative and Other Relationships (Textual)
Research and development										$ 46.0
Equity Method Investments, Expected Profit Share															50.00%
RITUXAN
Collaborative arrangements and non-collaborative arrangement transactions
Royalties Terms Of Collaboration Agreement															10% and 12% on sales of RITUXAN outside the U.S. and Canada, with the royalty period lasting 11 years
Collaborative and Other Relationships (Textual)
Period of collaboration agreement															11 years
Percentage of Co promotion Operating Profits first fifty million		30.00%													30.00%				30.00%
Percentage of share of Co promotion profits exceeding $50 million		40.00%				40.00%				40.00%					40.00%	40.00%	40.00%		40.00%	40.00%
Royalty Period For Substantially All Remaining Royalty Bearing Sales Rest Of World															expire through 2012
Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option One		39.00%													39.00%				39.00%
Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option One		40.00%													40.00%				40.00%
RITUXAN | Maximum
Collaborative and Other Relationships (Textual)
Percentage of royalties as per collaboration															12.00%
RITUXAN | Minimum
Collaborative and Other Relationships (Textual)
Percentage of royalties as per collaboration															10.00%
GAZYVA
Collaborative and Other Relationships (Textual)
Percentage of future development and commercialization expenses payable related to GAZYVA															35.00%
Percentage of Co promotion Operating Profits first fifty million		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
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%				39.00%
New Anti-CD20 | Maximum
Collaborative and Other Relationships (Textual)
Future percentage of co-promotion operating profits															38.00%
New Anti-CD20 | Minimum
Collaborative and Other Relationships (Textual)
Future percentage of co-promotion operating profits															30.00%
ELOCTATE
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN															$ 319.7	$ 58.4	$ 0.0
ELOCTATE | Swedish Orphan Biovitrum
Collaborative and Other Relationships (Textual)
Research and development															200.0
ALPROLIX | Swedish Orphan Biovitrum
Collaborative and Other Relationships (Textual)
Research and development															$ 175.0
Ocrelizumab
Collaborative and Other Relationships (Textual)
Reduction in royalty rate															50.00%
Ocrelizumab | Maximum
Collaborative and Other Relationships (Textual)
Future royalties percentage to be received on sale of ocrelizumab															24.00%
Ocrelizumab | Minimum
Collaborative and Other Relationships (Textual)
Future royalties percentage to be received on sale of ocrelizumab															13.50%
SOD1 | Ionis Pharmaceuticals
Collaborative and Other Relationships (Textual)
Research and development															$ 10.0
DMPK | Ionis Pharmaceuticals
Collaborative and Other Relationships (Textual)
Research and development													$ 12.0		2.8	14.0	10.0
Biogen's share of expense reflected within our consolidation statements of income															9.0	10.9	11.2
License Fee															70.0
Expected License Fee And Regulatory Milestone Payments		$ 130.0													130.0
Additional Milestone Payment		4.2											$ 59.0		4.2
SMNRx | Ionis Pharmaceuticals
Collaborative and Other Relationships (Textual)
Research and development														$ 29.0	42.8	57.3
Biogen's share of expense reflected within our consolidation statements of income															74.9	27.7	13.6
Estimated additional payments upon achievement of development and commercial milestones.						$ 325.0										325.0
Expected License Fee And Regulatory Milestone Payments						150.0										150.0
Additional Milestone Payment		92.0				100.0									92.0	100.0
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)
Additional Milestone Payment		75.0													75.0
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	$ 5.7				$ 8.6									$ 5.7	8.6			₩ 6.3	₩ 9.1	₩ 49.5
Percentage of equity interest to the portion of total capital stock	15.00%	9.00%													9.00%				9.00%		15.00%
Equity Method Investment Ownership Percentage Maximum	49.90%
Other revenues from external customers															$ 62.9	58.5	43.1
Samsung Biosimilar Agreement | Biosimilars
Collaborative and Other Relationships (Textual)
Additional Milestone Payment		$ 25.0													25.0
New Anti-CD20 | Third Party Anti CD-20 | Roche Group - Genentech
Collaborative and Other Relationships (Textual)
Research and development																	25.7
Hoechst
Collaborative and Other Relationships (Textual)
Decrease In Share Of Co Promotion Profits Due To Estimated Compensation Damages																	49.7
Research and development | Eisai
Collaborative and Other Relationships (Textual)
Amount recorded to reflect fair value of options granted									$ 17.7
Outside the U.S | AbbVie
Collaborative and Other Relationships (Textual)
Estimated additional payments upon achievement of development and commercial milestones.		12.0													12.0
Outside the U.S | RITUXAN | Roche Group - Genentech
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN															$ 69.4	78.3	38.7
Outside the U.S | Ocrelizumab
Collaborative and Other Relationships (Textual)
Future royalties percentage to be received on sale of ocrelizumab															3.00%
U.S
Collaborative and Other Relationships (Textual)
Revenue on sales in the rest of world for RITUXAN															$ 6,545.8	5,566.7	3,581.0
Other revenues from external customers															142.0	212.6	193.5
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															1,269.8	$ 1,117.1	$ 1,087.3
U.S | AbbVie
Collaborative and Other Relationships (Textual)
Estimated additional payments upon achievement of development and commercial milestones.		$ 20.0													$ 20.0