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Collaborative and Other Relationships - Equity Method Investments (Details)
$ in Millions, ₩ in Billions
1 Months Ended 3 Months Ended 9 Months Ended
Jun. 30, 2021
USD ($)
Nov. 30, 2018
USD ($)
Nov. 30, 2018
KRW (₩)
Sep. 30, 2021
USD ($)
Jun. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Jun. 30, 2020
USD ($)
Mar. 31, 2020
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
KRW (₩)
Dec. 31, 2020
USD ($)
Dec. 31, 2020
KRW (₩)
Jan. 31, 2020
USD ($)
Nov. 07, 2018
USD ($)
Schedule of Equity Method Investments [Line Items]                              
Income (loss) from equity method investments                 $ 17.2 $ (14.7)          
Income tax (benefit) expense       $ (25.9)   $ 240.8     (390.7) 979.0          
Collaboration profit sharing       21.2   73.0     74.5 166.5          
Total revenue       2,778.9   3,376.1     8,247.9 10,592.0          
ADUHELM                              
Schedule of Equity Method Investments [Line Items]                              
Total revenue       $ 0.3   $ 0.0     $ 2.0 $ 0.0          
Samsung Bioepis                              
Schedule of Equity Method Investments [Line Items]                              
Percentage of stake in entity           5.00%       5.00%          
Percentage of stake in entity maximum       49.90%         49.90%   49.90%        
Payments to acquire additional investment in equity method investment   $ 676.6 ₩ 759.5         $ 676.6              
Amortization of basis differences                             $ 675.0
Loss recorded on Samsung Bioepis joint venture       $ 1.1   $ (13.1)     $ 17.2 $ (12.7)          
Investment in Samsung Bioepis       586.1         586.1   ₩ 692.8 $ 620.2 ₩ 673.8    
Income tax (benefit) expense       31.2         $ 31.2            
Biogen share of co-promotion profits or losses                 50.00%            
Collaboration profit sharing       71.8   72.9     $ 210.2 200.1          
Eisai                              
Schedule of Equity Method Investments [Line Items]                              
Loss on research and development contracts terminated with Eisai             $ 45.0                
Income (loss) from equity method investments       50.6   33.8 $ 33.8   90.7            
Additional milestone payment           75.0                  
Eisai | ADUHELM                              
Schedule of Equity Method Investments [Line Items]                              
Income (loss) from equity method investments       45.0 $ 45.0       45.0            
Additional milestone payment $ 100.0     0.0   $ 0.0     100.0 $ 75.0          
Samsung Bioepis                              
Schedule of Equity Method Investments [Line Items]                              
Upfront and milestone payments made to collaborative partner               100.0              
Research and development expense               $ 63.0              
Prepaid research and development expenses                           $ 37.0  
Additional milestone payment       15.0                      
Accrued milestone payments       15.0                      
Estimated additional payments upon achievement of development and commercial milestones                 180.0            
Contract Option Exercise Fee                 60.0            
Due to Related Parties       $ 125.7         $ 125.7     $ 99.0      
Inventory | Samsung Bioepis                              
Schedule of Equity Method Investments [Line Items]                              
Equity method investment basis difference amortization period                 1 year 6 months            
Developed technology | Samsung Bioepis                              
Schedule of Equity Method Investments [Line Items]                              
Equity method investment basis difference amortization period                 15 years