Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Numerator:
Net (loss) income attributable to Biogen Inc.	$ (20.6)	$ 466.5	$ 396.4	$ 1,341.8
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	147,800	146,600	147,500	146,400
Effect of dilutive securities:
Dilutive potential common shares	0	500	1,400	400
Shares used in calculating diluted earnings per share	147,800	147,100	148,900	146,800
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0	500	1,200	400
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0	0	200	0

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