Share-Based Payments - Share-based Compensation Expense Included in Consolidation Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ (263.5)	$ (246.6)	$ (204.5)
Subtotal	273.6	258.8	211.3
Capitalized share-based compensation costs	(9.3)	(8.0)	(6.2)
Share-based compensation expense included in total cost and expense	264.3	250.8	205.1
Income tax effect	(49.2)	(46.7)	(33.5)
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	98.5	89.3	80.0
Selling, general and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	175.1	169.5	131.3
Share-based compensation expense included in net income attributable to Biogen Inc.
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 215.1	$ 204.1	$ 171.6

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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Amount, after tax, of expense for award under share-based payment arrangement.

+ References

No definition available.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(ii)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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- Definition

Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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