Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Numerator:
Net income attributable to Biogen Inc.	$ 634.8	$ 583.6	$ 875.3	$ 977.0
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	146,500	145,600	146,300	145,400
Effect of dilutive securities:
Dilutive potential common shares	200	300	400	500
Shares used in calculating diluted earnings per share	146,700	145,900	146,700	145,900
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	200	200	300	400
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0	100	100	100

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