Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Numerator:
Net income attributable to Biogen Inc.	$ 97.5	$ 634.8	$ 417.0	$ 875.3
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	147,700	146,500	147,400	146,300
Effect of dilutive securities:
Dilutive potential common shares	1,000	200	1,200	400
Shares used in calculating diluted earnings per share	148,700	146,700	148,600	146,700
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	800	200	1,000	300
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	200	0	200	100

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