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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Collaborative and Other Relationships Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 796.2			$ 473.1	$ 1,219.6	$ 910.4
Other	41.6			79.0	131.6	166.9
Collaboration profit (loss) sharing	26.5			(5.6)	47.3	(5.6)
Product, net	2,639.7			$ 2,466.0	5,019.8	$ 4,775.4
Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development					300.0
Additional milestone payments	410.0				410.0
iPerian
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments	490.0				490.0
Developmental Milestone Payment	60.0				$ 60.0
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	3.9				$ 9.8
AbbVie \| Rest of world
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration profit (loss) sharing					1.3
Ionis Pharmaceuticals [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License Fee			$ 75.0
Ionis Pharmaceuticals [Member] \| SPINRAZA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments	40.0				40.0
Ionis Pharmaceuticals [Member] \| SPINRAZA \| U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Product, net	194.8				241.2
Ionis Pharmaceuticals [Member] \| SPINRAZA \| Rest of world
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Product, net	$ 8.1				9.1
Acquired and in-licensed rights and patents \| SPINRAZA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in acquired and in-licensed rights and patents		$ 60.0			$ 50.0
Minimum \| Ionis Pharmaceuticals [Member] \| SPINRAZA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty payments per the collaboration					11.00%
Maximum \| Ionis Pharmaceuticals [Member] \| SPINRAZA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty payments per the collaboration					15.00%