Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Total revenue	$ 2,508.5	$ 2,778.9	$ 7,629.4	$ 8,247.9
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	469.5	511.8	1,707.4	1,449.6
Research and development	549.2	702.4	1,629.5	1,801.7
Selling, general and administrative	563.3	654.1	1,770.8	1,886.4
Amortization and impairment of acquired intangible assets	56.5	111.0	190.9	813.2
Collaboration profit (loss) sharing	45.3	21.2	(42.6)	74.5
(Gain) loss on fair value remeasurement of contingent consideration	(2.1)	(15.6)	(13.7)	(49.1)
Acquired in-process research and development	0.0	0.0	0.0	18.0
Restructuring charges	15.4	0.0	124.1	0.0
Gain on sale of building	(503.7)	0.0	(503.7)	0.0
Other (income) expense, net	(56.0)	502.9	(221.3)	913.4
Total cost and expense	1,137.4	2,487.8	4,641.4	6,907.7
Income before income tax expense and equity in loss of investee, net of tax	1,371.1	291.1	2,988.0	1,340.2
Income tax (benefit) expense	236.2	(25.9)	578.5	(390.7)
Equity in (income) loss of investee, net of tax	0.0	(1.1)	(2.6)	(17.2)
Net income	1,134.9	318.1	2,412.1	1,748.1
Net income (loss) attributable to noncontrolling interests, net of tax	0.2	(11.1)	(84.4)	560.2
Net income attributable to Biogen Inc.	$ 1,134.7	$ 329.2	$ 2,496.5	$ 1,187.9
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 7.86	$ 2.22	$ 17.12	$ 7.93
Diluted earnings per share attributable to Biogen Inc.	$ 7.84	$ 2.22	$ 17.07	$ 7.90
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	144,400	148,000	145,800	149,900
Diluted earnings per share attributable to Biogen Inc.	144,800	148,600	146,200	150,300
Product, net
Total revenue	$ 1,962.1	$ 2,205.7	$ 6,083.3	$ 6,653.4
Revenue from anti-CD20 therapeutic programs
Total revenue	416.9	415.4	1,252.6	1,244.4
Other
Total revenue	$ 129.5	$ 157.8	$ 293.5	$ 350.1