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Earnings Per Share (Tables)
 12 Months Ended
Dec. 31, 2017
Earnings Per Share [Abstract]  
Computation of basic and diluted net income per common share
The following table shows the computation of our basic and diluted net income per common share for the years ending December 31 (in millions, except per share amounts):
 
2017
 
2016
 
2015
Numerator:
 
 
 
 
 
Net income
$
502.0

 
$
257.1

 
$
205.3

 
 
 
 
 
 
Denominator:
 
 
 
 
 
Weighted average shares outstanding - basic
38.8

 
40.5

 
43.1

Effect of dilutive securities:
 
 
 
 
 
Equity compensation plans
0.6

 
0.4

 
0.7

Weighted average shares outstanding - diluted
39.4

 
40.9

 
43.8

Basic earnings per share
$
12.95

 
$
6.35

 
$
4.76

Diluted earnings per share
$
12.75

 
$
6.29

 
$
4.69