Earnings per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Numerator:
Net income attributable to Biogen Inc.	$ 1,556.1	$ 4,000.6	$ 5,888.5
Denominator:
Weighted average number of common shares outstanding (in shares)	149.1	160.9	187.1
Effect of dilutive securities:
Dilutive potential common shares (in shares)	0.5	0.4	0.3
Shares used in calculating diluted earnings per share (in shares)	149.6	161.3	187.4
Time-vested restricted stock units
Effect of dilutive securities:
Dilutive securities (in shares)	0.3	0.2	0.2
Market stock units
Effect of dilutive securities:
Dilutive securities (in shares)	0.1	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Dilutive securities (in shares)	0.1	0.1	0.0