Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Total revenue	$ 2,477.8	$ 2,431.0
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	661.0	629.3
Research and development	539.0	434.1
Acquired in-process research and development, upfront and milestone expense	34.0	200.7
Selling, general and administrative	607.3	572.5
Amortization and impairment of acquired intangible assets	136.5	111.8
Collaboration profit sharing/(loss reimbursement)	74.2	58.1
Business Combination, Contingent Consideration, Change in Contingent Consideration, Liability, Increase (Decrease)	20.5	9.6
Restructuring charges	7.9	35.3
Other (income) expense, net	19.7	68.4
Total cost and expense	2,100.1	2,119.8
Income before income tax (benefit) expense	377.7	311.2
Income tax (benefit) expense	58.2	70.7
Net income attributable to Biogen Inc.	$ 319.5	$ 240.5
Net income per share:
Basic earnings per share attributable to Biogen Inc. (in USD per share)	$ 2.17	$ 1.65
Diluted earnings per share attributable to Biogen Inc. (in USD per share)	$ 2.15	$ 1.64
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc. (in USD per share)	147.2	146.1
Diluted earnings per share attributable to Biogen Inc. (in USD per share)	148.4	146.6
Product revenue, net
Total revenue	$ 1,752.3	$ 1,726.5
Revenue from anti-CD20 therapeutic programs
Total revenue	419.1	378.2
Alzheimer's collaboration revenue
Total revenue	59.5	33.0
Contract manufacturing, royalty and other revenue
Total revenue	$ 246.9	$ 293.3