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Earnings per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) 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
Numerator:
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
Denominator:
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
Effect of dilutive securities:
Dilutive potential common shares (in shares)									0.4	0.3	0.4
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
Time-vested restricted stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan (in shares)									0.2	0.2	0.3
Market stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan (in shares)									0.1	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock options and employee stock purchase plan (in shares)									0.1	0.0	0.0

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