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Earnings per Share (Tables)
 12 Months Ended
Dec. 31, 2019
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)
2019
 
2018
 
2017
Numerator:
 
 
 
 
 
Net income attributable to Biogen Inc.
$
5,888.5

 
$
4,430.7

 
$
2,539.1

Denominator:
 
 
 
 
 
Weighted average number of common shares outstanding
187.1

 
204.9

 
212.6

Effect of dilutive securities:
 
 
 
 
 
Stock options and employee stock purchase plan
—

 
—

 
0.1

Time-vested restricted stock units
0.2

 
0.3

 
0.2

Market stock units
0.1

 
0.1

 
0.1

Performance stock units settled in stock
—

 
—

 
—

Dilutive potential common shares
0.3

 
0.4

 
0.4

Shares used in calculating diluted earnings per share
187.4

 
205.3

 
213.0