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Earnings per Share
 9 Months Ended
Sep. 30, 2019
Earnings Per Share [Abstract]  
Earnings per Share Earnings per Share
Basic and diluted earnings per share are calculated as follows:
 
For the Three Months
Ended September 30,
 
For the Nine Months
Ended September 30,
(In millions)
2019
 
2018
 
2019
 
2018
Numerator:
 
 
 
 
 
 
 
Net income attributable to Biogen Inc.
$
1,545.9

 
$
1,444.4

 
$
4,448.8

 
$
3,483.9

Denominator:
 
 
 
 
 
 
 
Weighted average number of common shares outstanding
184.0

 
201.4

 
190.3

 
206.6

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

 
—

 
—

 
—

Time-vested restricted stock units
0.1

 
0.4

 
0.1

 
0.3

Market stock units
0.1

 
0.1

 
0.1

 
0.1

Performance stock units settled in stock
—

 
—

 
—

 
—

Dilutive potential common shares
0.2

 
0.5

 
0.2

 
0.4

Shares used in calculating diluted earnings per share
184.2

 
201.9

 
190.5

 
207.0


Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.