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Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 6 Months Ended
Feb. 29, 2012
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Schedule of Equity Method Investments [Line Items]          
Collaboration profit (loss) sharing   $ 39.2 $ 26.5 $ 81.7 $ 47.3
Other revenues   $ (3,356.5) (3,078.4) $ (6,487.6) (5,889.1)
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%        
Payments to acquire additional investment in equity method investment   $ 700.0      
Biogen share of co-promotion profits or losses       50.00%  
Collaboration profit (loss) sharing   39.7 25.2 $ 83.5 46.0
Collaborative arrangement | Samsung Bioepis          
Schedule of Equity Method Investments [Line Items]          
Other revenues   $ (14.7) $ (12.7) $ (32.6) $ (14.9)