Share-Based Payments - Share-Based Compensation Expense Included In Statements Of Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Share-based Compensation Expense included in consolidated statements of income
Subtotal	$ 78.1	$ 72.9	$ 162.2	$ 144.3
Capitalized share-based compensation costs	(3.2)	(2.3)	(6.3)	(5.6)
Share-based compensation expense included in total cost and expense	74.9	70.6	155.9	138.7
Income tax effect	(14.9)	(13.4)	(30.9)	(26.3)
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	30.1	27.3	62.2	54.7
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	48.0	45.6	100.0	89.6
Share-based compensation expense included in net income attributable to Biogen Inc.
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 60.0	$ 57.2	$ 125.0	$ 112.4

Share based compensation expense included in costs and expenses.

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- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479983/718-10-S45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
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Amount, after tax, of expense for award under share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
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-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

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Amount of cost capitalized for award under share-based payment arrangement.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
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-Name Accounting Standards Codification
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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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