Earnings per Share (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net income attributable to Biogen Inc	$ 927,276	$ 714,506	$ 1,749,817	$ 1,194,462
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	235,286	236,661	235,122	236,729
Effect of dilutive securities:
Dilutive potential common shares	400	700	600	900
Shares used in calculating diluted earnings per share	235,718	237,401	235,671	237,634
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	400	300	500
Market stock units
Effect of dilutive securities:
Stock units	100	200	200	300