Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Numerator:
Net income attributable to Biogen Inc.	$ 466.5	$ 388.5	$ 1,341.8	$ 1,365.5
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	146,600	145,700	146,400	145,500
Effect of dilutive securities:
Dilutive potential common shares	500	400	400	500
Shares used in calculating diluted earnings per share	147,100	146,100	146,800	146,000
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	500	300	400	400
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0	100	0	100

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