Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total Revenues	$ 14,377.9	$ 13,452.9	$ 12,273.9
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	1,955.4	1,816.3	1,630.0
Research and development	2,280.6	2,597.2	2,253.6
Selling, general and administrative	2,374.7	2,106.3	1,933.9
Amortization and impairment of acquired intangible assets	489.9	747.3	814.7
Collaboration profit (loss) sharing	241.6	185.0	112.3
Loss on divestiture of Hillerød, Denmark manufacturing operations	55.3	0.0	0.0
Acquired in-process research and development	0.0	112.5	120.0
(Gain) loss on fair value remeasurement of contingent consideration	(63.7)	(12.3)	62.7
Restructuring Charges	1.5	12.0	0.9
Total cost and expenses	7,335.3	7,564.3	6,928.1
Income from operations	7,042.6	5,888.6	5,345.8
Other income (expense), net	83.3	11.0	(217.0)
Income before income tax expense and equity in loss of investee, net of tax	7,125.9	5,899.6	5,128.8
Income tax expense	1,158.0	1,425.6	2,458.7
Equity in loss of investee, net of tax	79.4	0.0	0.0
Net income	5,888.5	4,474.0	2,670.1
Net income (loss) attributable to noncontrolling interests, net of tax	0.0	43.3	131.0
Net income attributable to Biogen Inc.	$ 5,888.5	$ 4,430.7	$ 2,539.1
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 31.47	$ 21.63	$ 11.94
Diluted earnings per share attributable to Biogen Inc.	$ 31.42	$ 21.58	$ 11.92
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	187.1	204.9	212.6
Diluted earnings per share attributable to Biogen Inc.	187.4	205.3	213.0
Product, net
Total Revenues	$ 11,379.8	$ 10,886.8	$ 10,354.7
Revenues from anti-cd20 therapeutic programs
Total Revenues	2,290.4	1,980.2	1,559.2
Other revenue
Total Revenues	$ 707.7	$ 585.9	$ 360.0