Subsequent Events (Details) - Subsequent Event	1 Months Ended	11 Months Ended
Feb. 28, 2020 USD ($) product_target $ / shares	Dec. 31, 2020 USD ($) shares
Subsequent Event [Line Items]
Agreement restriction, ownership percentage (less than)	5.00%
Biogen MA Inc \| Collaboration and license agreement
Subsequent Event [Line Items]
Number of neurological disease gene targets \| product_target	12
Number of product targets selected \| product_target	3
Number of additional neurological disease gene targets \| product_target	9
Target selection period	5 years
Biogen MA Inc \| Collaboration and license agreement \| Forecast
Subsequent Event [Line Items]
Proceeds from collaborators		$ 125,000,000.0
Biogen MA Inc \| Collaboration and license agreement \| Pre-approval Milestone
Subsequent Event [Line Items]
Maximum milestone payment receivable	$ 925,000,000.0
Biogen MA Inc \| Collaboration and license agreement \| Sales-based Milestone
Subsequent Event [Line Items]
Maximum milestone payment receivable	1,445,000,000.0
Biogen MA Inc \| Collaboration and license agreement \| Achievement of specified research, clinical development, regulatory and first commercial sale milestones
Subsequent Event [Line Items]
Maximum milestone payment receivable	$ 2,370,000,000
Biogen MA Inc \| Stock Purchase Agreement
Subsequent Event [Line Items]
Agreement restriction expiration period	3 years
Agreement restriction, percentage of shares held	50.00%
Agreement provision, expiration period	2 years
Agreement provision, ownership percentage (less than)	5.00%
Biogen MA Inc \| Stock Purchase Agreement \| Forecast
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares		24,420,157
Sale of stock, price per share (in dollars per share) \| $ / shares	$ 9.2137
Consideration received from sale of stock		$ 225,000,000.0

Collaboration Agreement, Number Of Additional Product Targets

No definition available.

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- Definition

Collaboration Agreement, Target Selection Period

+ References

No definition available.

+ Details

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- Definition

Collaborative Arrangement, Agreement Restriction, Percentage Of Shares Held

+ References

No definition available.

+ Details

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- Definition

Collaborative Arrangement, Maximum Milestone Payment Receivable

+ References

No definition available.

+ Details

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- Definition

Collaborative Arrangement, Number Of Product Targets

+ References

No definition available.

+ Details

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- Definition

Collaborative Arrangement, Number Of Product Targets Selected

+ References

No definition available.

+ Details

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- Definition

Collaborative Arrangement, Standstill Restriction, Ownership Threshold Percentage Ownership Percentage

+ References

No definition available.

+ Details

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- Definition

Collaborative Arrangement, Standstill Restriction Period

+ References

No definition available.

+ Details

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- Definition

Collaborative Arrangement, Voting Provisions Expiration Period

+ References

No definition available.

+ Details

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- Definition

Collaborative Arrangement, Voting Provisions, Ownership Threshold Percentage

+ References

No definition available.

+ Details

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- Definition

Cash received from collaborators during the current period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3536-108585

+ Details

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- Definition

Cash received on stock transaction after deduction of issuance costs.

+ References

No definition available.

+ Details

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- Definition

The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.

+ References

No definition available.

+ Details

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- Definition

Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

+ References

No definition available.

+ Details

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- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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- Details

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- Details

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- Details

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- Details

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