Earnings per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Numerator:
Net income attributable to Biogen Inc.	$ 3,046.9	$ 1,556.1	$ 4,000.6
Denominator:
Weighted average number of common shares outstanding (in shares)	145,300	149,100	160,900
Effect of dilutive securities:
Dilutive potential common shares (in shares)	700	500	400
Shares used in calculating diluted earnings per share (in shares)	146,000	149,600	161,300
Time-vested restricted stock units
Effect of dilutive securities:
Dilutive securities (in shares)	500	300	200
Market stock units
Effect of dilutive securities:
Dilutive securities (in shares)	100	100	100
Performance stock units settled in stock
Effect of dilutive securities:
Dilutive securities (in shares)	100	100	100