Collaborative and Other Relationships Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Mar. 31, 2012	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Jan. 31, 2007
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 529.0		$ 519.9		$ 1,439.4	$ 1,471.1
Investments and other assets	1,239.6				1,239.6		$ 1,107.6
Other	98.6		49.0		265.5	156.6
Collaboration profit (loss) sharing	4.7		0.0		$ (0.9)	0.0
Sobi rate to Biogen on net sales in the Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method					Royalty
Base rate after first commercial sale					12.00%
Rate during the reimbursement period					Base Rate plus 5%
Biogen rate to Sobi on net sales in the Biogen North America Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method					Royalty
Rate during the reimbursement period					Base Rate less 5%
Biogen rate to Sobi on net sales in the Biogen North America Territory | Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Base rate after first commercial sale					12.00%
Biogen rate to Sobi on net sales in the Biogen Direct Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method					Royalty
Rate during the reimbursement period					Base Rate less 5%
Biogen rate to Sobi on net sales in the Biogen Direct Territory | Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Base rate after first commercial sale					17.00%
Biogen rate to Sobi on net revenue from the Biogen Distributor Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method					Net Revenue Share
Rate during the reimbursement period					Base Rate less 15%
Biogen rate to Sobi on net revenue from the Biogen Distributor Territory | Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Base rate after first commercial sale					50.00%
Biogen Hemophilia
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of stock acquired								100.00%
University of Pennsylvania
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development		$ 20.0
Investments and other assets		$ 15.0
Future research and development commitment	47.5				$ 47.5
Potential future milestone payments	2,000.0				$ 2,000.0
Swedish Orphan Biovitrum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement recognition period					10 years
Reimbursement cost achieving period					6 years
Time period for paying remaining balance due					90 days
Swedish Orphan Biovitrum | ELOCTATE
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development					$ 151.0
Consideration per product					10.0
Swedish Orphan Biovitrum | ALPROLIX
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development					129.0
Consideration per product					$ 10.0
AbbVie
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen share of co-promotion profits or losses					50.00%
AbbVie | U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Other					$ 13.5
AbbVie | E.U.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration profit (loss) sharing					2.7
Ionis Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development				$ 29.0
License Fee	75.0
Ionis Pharmaceuticals, Inc. | Nusinersen
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	25.7		$ 10.6		58.9	$ 23.9
Potential future milestone payments	150.0				150.0
Samsung Biosimilar Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration profit (loss) sharing	7.4				1.8
Acquired and in-licensed rights and patents | ALPROLIX
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in acquired and in-licensed rights and patents					26.5
Acquired and in-licensed rights and patents | ZINBRYTA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in acquired and in-licensed rights and patents					32.0
Acquired and in-licensed rights and patents | ZINBRYTA | U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in acquired and in-licensed rights and patents					20.0
Acquired and in-licensed rights and patents | ZINBRYTA | E.U.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in acquired and in-licensed rights and patents					12.0
Acquired and in-licensed rights and patents | Biosimilars
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in acquired and in-licensed rights and patents	$ 25.0				$ 50.0
Royalty revenues | Swedish Orphan Biovitrum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen effective royalty rate					0.14
Cost of sales | Swedish Orphan Biovitrum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen effective royalty rate					0.11