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Business Acquisition (Details Textual) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Mar. 31, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Business Acquisition [Line Items] |  |  |  |  |
Acquired in-process research and development |  | $ 0.0 | $ 0.0 | $ 18.0 | $ 75.0
BIIB118 |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquired in-process research and development | $ 75.0 |  |  |  |
Estimated additional payments upon achievement of development and commercial milestones | $ 635.0 |  |  |  |

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- Definition Estimated additional payments upon achievement of development and commercial milestones. + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

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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 No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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- Definition The amount of purchased research and development assets that are acquired in a business combination have no alternative future use and are therefore written off in the period of acquisition. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |