Earnings per Share (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Net income attributable to Biogen Inc	$ 965,622	$ 856,858	$ 2,715,439	$ 2,051,320
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	232,191	236,217	234,134	236,641
Effect of dilutive securities:
Dilutive potential common shares	400	800	600	800
Shares used in calculating diluted earnings per share	232,612	236,972	234,659	237,449
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	100	100	100	100
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	200	500	300	500
Market stock units
Effect of dilutive securities:
Stock units	100	200	200	200