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Quarterly Financial Data (Unaudited) (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]                      
Revenues $ 2,852.6 $ 3,376.1 $ 3,681.6 $ 3,534.3 $ 3,671.3 $ 3,600.1 $ 3,616.7 $ 3,489.8 $ 13,444.6 $ 14,377.9 $ 13,452.9
Gross profit [1] 2,361.9 2,927.0 3,270.5 3,080.0 3,224.2 3,170.1 3,140.4 2,887.8 11,639.4 12,422.5  
Net income 357.6 703.9 1,606.5 1,392.6 1,439.7 1,545.9 1,494.1 1,408.8 4,060.5 5,888.5 4,474.0
Net income attributable to Biogen Inc. $ 357.9 $ 701.5 $ 1,542.1 $ 1,399.1 $ 1,439.7 $ 1,545.9 $ 1,494.1 $ 1,408.8 $ 4,000.6 $ 5,888.5 $ 4,430.7
Net income per share:                      
Basic earnings per share attributable to Biogen Inc. (in dollars per share) $ 2.33 $ 4.47 $ 9.60 $ 8.10 $ 8.10 $ 8.40 $ 7.85 $ 7.17 $ 24.86 $ 31.47 $ 21.63
Diluted earnings per share attributable to Biogen Inc. (in dollars per share) $ 2.32 $ 4.46 $ 9.59 $ 8.08 $ 8.08 $ 8.39 $ 7.85 $ 7.15 $ 24.80 $ 31.42 $ 21.58
Weighted-average shares used in calculating:                      
Basic earnings per share attributable to Biogen Inc. (in shares) 153.7 156.9 160.6 172.8 177.8 184.0 190.3 196.6 160.9 187.1 204.9
Diluted earnings per share attributable to Biogen Inc. (in shares) 154.0 157.2 160.9 173.1 178.2 184.2 190.4 197.0 161.3 187.4 205.3
Product, net                      
Selected Quarterly Financial Information [Abstract]                      
Revenues $ 2,301.6 $ 2,690.3 $ 2,795.7 $ 2,904.6 $ 2,924.8 $ 2,894.7 $ 2,880.3 $ 2,680.0 $ 10,692.2 $ 11,379.8 $ 10,886.8
Revenues from anti-CD20 therapeutic programs                      
Selected Quarterly Financial Information [Abstract]                      
Revenues 419.0 560.1 478.3 520.4 600.8 595.8 576.4 517.4 1,977.8 2,290.4 1,980.2
Other                      
Selected Quarterly Financial Information [Abstract]                      
Revenues $ 132.0 $ 125.7 $ 407.6 $ 109.3 $ 145.7 $ 109.6 $ 160.0 $ 292.4 $ 774.6 $ 707.7 $ 585.9
[1] (1) Gross profit is calculated as total revenues less cost of sales, excluding amortization and impairment of acquired intangible assets