Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Total revenues	$ 3,616.7	$ 3,356.5	$ 7,106.5	$ 6,487.6
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	476.3	421.0	1,078.3	867.0
Research and development	484.8	981.0	1,048.5	1,477.7
Selling, general and administrative	587.6	516.2	1,155.3	1,017.5
Amortization and impairment of acquired intangible assets	70.1	107.4	138.3	211.3
Collaboration profit (loss) sharing	63.5	39.2	121.6	81.7
Loss on assets and liabilities held for sale	(2.3)	0.0	113.2	0.0
(Gain) loss on fair value remeasurement of contingent consideration	(20.0)	1.9	(8.5)	(3.7)
Restructuring charges	0.8	1.6	1.2	3.2
Acquired in-process research and development	0.0	75.0	0.0	85.0
Total cost and expenses	1,660.8	2,143.3	3,647.9	3,739.7
Income from operations	1,955.9	1,213.2	3,458.6	2,747.9
Other income (expense), net	(197.4)	(34.5)	159.9	(75.5)
Income before income tax expense and equity in loss of investee, net of tax	1,758.5	1,178.7	3,618.5	2,672.4
Income tax expense	248.1	263.7	670.6	586.2
Equity in loss of investee, net of tax	16.3	0.0	45.0	0.0
Net income	1,494.1	915.0	2,902.9	2,086.2
Net income (loss) attributable to noncontrolling interests, net of tax	0.0	48.4	0.0	46.7
Net income attributable to Biogen Inc.	$ 1,494.1	$ 866.6	$ 2,902.9	$ 2,039.5
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 7.85	$ 4.18	$ 15.01	$ 9.75
Diluted earnings per share attributable to Biogen Inc.	$ 7.85	$ 4.18	$ 14.99	$ 9.73
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	190.3	207.1	193.4	209.2
Diluted earnings per share attributable to Biogen Inc.	190.4	207.3	193.7	209.5
Product, net
Total revenues	$ 2,880.3	$ 2,757.5	$ 5,560.3	$ 5,281.0
Revenues from anti-CD20 therapeutic programs
Total revenues	576.4	490.4	1,093.8	933.6
Other
Total revenues	$ 160.0	$ 108.6	$ 452.4	$ 273.0