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Collaborative and Other Relationships - Summary of Activity Related to Aducanumab Collaboration (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2020 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |
Biogen's share of reimbursement from Eisai of ADUHELM milestone payments reflected in collaboration profit (loss) sharing in our consolidated statements of income |  | $ 2.6 | $ 34.9 | $ 3.3
Eisai |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |
Expense reflected within statements of income |  | 173.6 | 161.5 | 109.6
Total ADUHELM Collaboration third party milestones |  | 0.0 | 100.0 | 75.0
Biogen's share of reimbursement from Eisai of ADUHELM milestone payments reflected in collaboration profit (loss) sharing in our consolidated statements of income | $ 33.8 | 0.0 | 45.0 | 33.8
Aducanumab | Research and development | Eisai |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |
Expense incurred by the collaboration |  | 149.4 | 183.7 | 152.0
Expense reflected within statements of income |  | 82.2 | 101.1 | 83.6
Aducanumab | Selling, general and administrative | Eisai |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |
Expense incurred by the collaboration |  | 134.2 | 562.3 | 353.0
Expense reflected within statements of income |  | $ 71.5 | $ 301.4 | $ 193.7

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