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Quarterly Financial Data (Unaudited) (Details Textual) - USD ($) shares in Millions, $ in Millions	1 Months Ended		3 Months Ended							12 Months Ended
	May 31, 2018	Oct. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 05, 2018
Schedule Of Development Milestone And Collaboration [Line Items]
Unrealized gain (loss) on strategic investments			$ (12.2)	$ 141.2	$ 5.4	$ (6.4)
Amortization of Intangible Assets										$ 381.2	$ 455.3	$ 373.4
GILTI			(544.6)				$ 0.0			(544.6)	0.0
Loss (gain) on fair value remeasurement of contingent consideration										12.3	(62.7)	(14.8)
Research and development										2,597.2	2,253.6	1,973.3
Acquired in-process research and development										112.5	120.0	0.0
Payments to Noncontrolling Interests										36.4	134.1	0.0
Net income (loss) attributable to noncontrolling interests, net of tax										43.3	131.0	(7.1)
Income tax benefit (expense)										1,425.6	2,458.7	$ 1,237.3
Bristol-Myers Squibb
Schedule Of Development Milestone And Collaboration [Line Items]
Research and development								$ 300.0		97.0
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			68.7	80.3
Upfront And Milestone Payments Made To Collaborative Partner							28.0
Ionis Pharmaceuticals
Schedule Of Development Milestone And Collaboration [Line Items]
Research and development expense on 2018 Ionis Agreement					486.2
Research and development					324.1								$ 10.0
Premium on purchase of common stock														$ 162.1
Investment in common stock, shares purchased														11.5
Upfront And Milestone Payments Made To Collaborative Partner							25.0
Neurimmune
Schedule Of Development Milestone And Collaboration [Line Items]
Payments to Noncontrolling Interests	$ 50.0	$ 150.0
Additional reduction in royalty rate payable on commercial sales	5.00%
Net income (loss) attributable to noncontrolling interests, net of tax							150.0
Reduction in royalty rate payable on commercial sales		15.00%
TECFIDERA
Schedule Of Development Milestone And Collaboration [Line Items]
Amortization of Intangible Assets			176.8						$ 328.2
Karyopharm
Schedule Of Development Milestone And Collaboration [Line Items]
Acquired in-process research and development						$ 10.0
Remedy Pharmaceutical
Schedule Of Development Milestone And Collaboration [Line Items]
Acquired in-process research and development								$ 120.0
Pfizer
Schedule Of Development Milestone And Collaboration [Line Items]
Acquired in-process research and development					$ 75.0
AliveGen
Schedule Of Development Milestone And Collaboration [Line Items]
Acquired in-process research and development				27.5
In-process research and development
Schedule Of Development Milestone And Collaboration [Line Items]
Amortization of Intangible Assets				189.3
TGN
Schedule Of Development Milestone And Collaboration [Line Items]
Loss (gain) on fair value remeasurement of contingent consideration			80.6	89.6
TGN | In-process research and development
Schedule Of Development Milestone And Collaboration [Line Items]
Amortization of Intangible Assets				$ 129.3
2017 Tax Act
Schedule Of Development Milestone And Collaboration [Line Items]
GILTI			135.8							135.8
Income tax benefit (expense)											1,173.6
Transition toll tax liabilities			$ 697.0				$ 989.6			$ 697.0	$ 989.6

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