Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net income attributable to Biogen Inc	$ 1,032.9	$ 965.6	$ 3,053.6	$ 2,715.4
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	218.9	232.2	219.0	234.1
Effect of dilutive securities:
Dilutive potential common shares	0.5	0.4	0.4	0.6
Shares used in calculating diluted earnings per share	219.4	232.6	219.4	234.7
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	0.1	0.1	0.1	0.1
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.2	0.2	0.2	0.3
Market stock units
Effect of dilutive securities:
Stock units	0.2	0.1	0.1	0.2