Fair Value Measurements (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	Feb. 28, 2015	Mar. 31, 2012	Dec. 31, 2010	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 12, 2015	Mar. 04, 2008
Business Acquisition, Contingent Consideration [Line Items]
Fair value measurements, changes in valuation techniques				0	0
Maximum contingent consideration in the form of development and approval milestones				$ 1,100.0
Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Milestone converted to short-term obligation					$ 7.9
Additions	$ 274.5
Contingent consideration obligations				259.0	258.9
Maximum contingent consideration in the form of development and approval milestones				$ 400.0			$ 450.0
Discount rate used for net cash outflow projections for fair value measurement	2.00%			2.40%
In-process research and development							$ 424.6
Stromedix, Inc.
Business Acquisition, Contingent Consideration [Line Items]
Additions		$ 122.2
Contingent consideration obligations				$ 162.4	133.2
Maximum contingent consideration in the form of development and approval milestones				$ 344.0
Discount rate used for net cash outflow projections for fair value measurement				2.40%
Biogen Idec International Neuroscience GmbH
Business Acquisition, Contingent Consideration [Line Items]
Additions			$ 81.2
Contingent consideration obligations				$ 102.2	75.5
Maximum contingent consideration in the form of development and approval milestones				$ 355.0
Discount rate used for net cash outflow projections for fair value measurement				2.80%
Milestone payments made during period				$ 6.7
6.875% Senior Notes due 2018
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes				6.875%				6.875%
6.875% Senior Notes aggregate principal amount								$ 550.0
2.90% Senior Notes due 2020
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes				2.90%		2.90%
6.875% Senior Notes aggregate principal amount						$ 1,500.0
3.625% Senior Notes due 2022
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes				3.625%		3.625%
6.875% Senior Notes aggregate principal amount						$ 1,000.0
4.05% Senior Notes due 2025
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes				4.05%		4.05%
6.875% Senior Notes aggregate principal amount						$ 1,750.0
5.20% Senior Notes due 2045
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes				5.20%		5.20%
6.875% Senior Notes aggregate principal amount						$ 1,750.0
Other long-term liabilities
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations				$ 279.0	$ 246.8
Accrued expenses and other | Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations				147.9
Accrued expenses and other | Stromedix, Inc.
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations				76.7
Accrued expenses and other | Biogen Idec International Neuroscience GmbH
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations				$ 20.0