Fair Value Measurements (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended					12 Months Ended
	Feb. 28, 2015	Dec. 31, 2015	Sep. 30, 2014	Mar. 31, 2012	Sep. 30, 2011	Dec. 31, 2010	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 12, 2015
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations		$ 301.3					$ 301.3	$ 200.0
Fair value measurements, changes in valuation techniques							0	0
(Gain) loss on fair value remeasurement of contingent consideration			$ 49.4				$ (30.5)	$ 38.9	$ 0.5
Additions							274.5	0.0
Potential future milestone payments		2,800.0					2,800.0
Maximum contingent consideration in the form of development and approval milestones		1,300.0					1,300.0
Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Additions	$ 274.5
Potential future milestone payments										$ 450.0
Contingent consideration obligations		$ 297.5					297.5
Discount rate used for net cash outflow projections for fair value measurement	2.00%	3.00%
In-process research and development										$ 424.6
Stromedix, Inc.
Business Acquisition, Contingent Consideration [Line Items]
Additions				$ 122.2
Contingent consideration obligations		$ 131.5					131.5	130.5
Maximum contingent consideration in the form of development and approval milestones		419.0					$ 419.0
Discount rate used for net cash outflow projections for fair value measurement							2.00%
Biogen Dompe SRL and Biogen Dompe Switzerland GmbH
Business Acquisition, Contingent Consideration [Line Items]
Additions					$ 38.8
Contingent consideration obligations		0.0					$ 0.0	15.5
Milestone payments made during period							14.5
Biogen Idec International Neuroscience GmbH
Business Acquisition, Contingent Consideration [Line Items]
Additions						$ 81.2
Contingent consideration obligations		77.0					77.0	69.5
Maximum contingent consideration in the form of development and approval milestones		$ 365.0					$ 365.0
Discount rate used for net cash outflow projections for fair value measurement							3.00%
6.875% Senior Notes due 2018
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes		6.875%					6.875%
2.90% Senior Notes due 2020
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes		2.90%					2.90%
3.625% Senior Notes due 2022
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes		3.625%					3.625%
4.05% Senior Notes due 2025
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes		4.05%					4.05%
5.20% Senior Notes due 2045
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes		5.20%					5.20%
Other long-term liabilities
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations		$ 301.3					$ 301.3	$ 200.0
Accrued expenses and other | Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations		$ 197.2					$ 197.2