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Business Acquisition (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Business Acquisition [Line Items]
Acquired in-process research and development		$ 18.0	$ 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

X

- Definition

Estimated additional payments upon achievement of development and commercial milestones.

+ References

No definition available.

+ Details

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X

- 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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X

- 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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- Details

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