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Share-Based Payments - Share-based Compensation Expense Included in Consolidation Statements of Income (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |
Share-based compensation expense | $ (204.5) | $ (191.2) | $ (168.7)
Subtotal | 211.3 | 225.4 | 181.6
Capitalized share-based compensation costs | (6.2) | (8.9) | (11.5)
Share-based compensation expense included in total cost and expenses | 205.1 | 216.5 | 170.1
Income tax effect | (33.5) | (35.7) | (27.5)
Research and development |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |
Share-based compensation expense | 80.0 | 77.1 | 75.8
Selling, general and administrative |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |
Share-based compensation expense | 131.3 | 148.3 | 105.8
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 | $ 171.6 | $ 180.8 | $ 142.6

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