Fair Value Measurements (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended										12 Months Ended
	Feb. 28, 2015	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2012	Dec. 31, 2010	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015	Feb. 12, 2015
Business Acquisition, Contingent Consideration [Line Items]
Fair value measurements, changes in valuation techniques												no	0
Payments												$ 0.0	$ 101.5
Loss (gain) on fair value remeasurement of contingent consideration		$ (2.6)	$ 57.8	$ 20.0	$ (11.5)	$ (79.3)	$ 87.9	$ (1.9)	$ 5.6			63.7	12.3	$ (62.7)
Maximum contingent consideration in the form of development and approval milestones		735.0										735.0
Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Additions	$ 274.5
Contingent consideration obligations		244.6				246.6						244.6	246.6
Maximum contingent consideration in the form of development and approval milestones		400.0										400.0
In-process research and development																$ 424.6
Stromedix, Inc.
Business Acquisition, Contingent Consideration [Line Items]
Payments													(81.5)
Additions										$ 122.2
Contingent consideration obligations						83.0							83.0
In-process research and development										$ 219.2
Biogen Idec International Neuroscience GmbH
Business Acquisition, Contingent Consideration [Line Items]
Additions											$ 81.2
Contingent consideration obligations		101.5				80.2						101.5	80.2
Maximum contingent consideration in the form of development and approval milestones		$ 335.0										$ 335.0
2.90% Senior Notes due 2020
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes		2.90%										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%			3.625%
4.05% Senior Notes due 2025
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes		4.05%										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%			5.20%
Other long-term liabilities
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations		$ 197.7				$ 265.0						$ 197.7	$ 265.0
Accrued expenses and other | Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations		$ 148.5										$ 148.5
Discount rate | Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Fair value measurement and measurement inputs, recurring and nonrecurring		2.10%										2.10%
Discount rate | Biogen Idec International Neuroscience GmbH
Business Acquisition, Contingent Consideration [Line Items]
Fair value measurement and measurement inputs, recurring and nonrecurring		2.30%										2.30%
BG00011
Business Acquisition, Contingent Consideration [Line Items]
Loss (gain) on fair value remeasurement of contingent consideration			$ 61.2