Earnings per Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Basic and diluted earnings per share

Basic and diluted earnings per share are calculated as follows:

	For the Years Ended December 31,
(In millions)	2017		2016		2015
Numerator:
Net income attributable to Biogen Inc.	$	2,539.1	$	3,702.8	$	3,547.0
Denominator:
Weighted average number of common shares outstanding	212.6		218.4		230.7
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.1		0.1		0.1
Time-vested restricted stock units	0.2		0.2		0.3
Market stock units	0.1		0.1		0.1
Dilutive potential common shares	0.4		0.4		0.5
Shares used in calculating diluted earnings per share	213.0		218.8		231.2