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Quarterly Financial Data (Unaudited) (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended									12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2013	Mar. 31, 2012	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2012
Schedule Of Development Milestone And Collaboration [Line Items]
Amortization of Intangible Assets											$ 814.7	$ 385.6	$ 382.6
Research and development											2,253.6	1,973.3	2,012.8
Acquired in-process research and development											120.0	0.0	0.0
Net income (loss) attributable to noncontrolling interests, net of tax											131.0	(7.1)	46.2
Income tax benefit (expense)											2,458.7	1,237.3	1,161.6
TECFIDERA litigation settlement charge						$ 454.8					0.0	454.8	0.0
Bristol-Myers Squibb [Member]
Schedule Of Development Milestone And Collaboration [Line Items]
Research and development				$ 300.0
iPerian
Schedule Of Development Milestone And Collaboration [Line Items]
Developmental Milestone Payment				60.0
Alkermes
Schedule Of Development Milestone And Collaboration [Line Items]
Research and development		$ 50.0									80.3
Upfront And Milestone Payments Made To Collaborative Partner		28.0									28.0
Ionis Pharmaceuticals, Inc.
Schedule Of Development Milestone And Collaboration [Line Items]
Research and development									$ 75.0		12.0		10.0
Upfront And Milestone Payments Made To Collaborative Partner		25.0							$ 100.0		25.0			$ 30.0
License Fee											70.0
Eisai
Schedule Of Development Milestone And Collaboration [Line Items]
Research and development											263.4	50.0
Neurimmune
Schedule Of Development Milestone And Collaboration [Line Items]
Net income (loss) attributable to noncontrolling interests, net of tax		150.0
Reduction in royalty rate payable on commercial sales	15.00%
Cambridge, MA
Schedule Of Development Milestone And Collaboration [Line Items]
Accelerated depreciation						$ 14.0	$ 15.7	$ 15.8				45.5
TECFIDERA
Schedule Of Development Milestone And Collaboration [Line Items]
Amortization of Intangible Assets		$ 30.8	$ 30.4	29.4	$ 353.6						$ 444.2
SPINRAZA | Ionis Pharmaceuticals, Inc.
Schedule Of Development Milestone And Collaboration [Line Items]
Research and development										$ 29.0		$ 35.3	$ 42.8
License Fee							$ 75.0
Remedy Pharmaceutical [Member]
Schedule Of Development Milestone And Collaboration [Line Items]
Acquired in-process research and development				$ 120.0

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