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Stock Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Based Compensation Activity The following table shows a summary of RSU activity during the three and nine months ended September 30, 2018 and 2017:
 
Three Months Ended 
 September 30,
 
Nine Months Ended 
 September 30,
 
2018
 
2017
 
2018
 
2017
RSU compensation expense
$
10.9

 
$
6.3

 
$
24.7

 
$
17.1

Income tax benefit
(1.9
)
 
(2.2
)
 
(4.2
)
 
(6.0
)
RSU compensation expense, net of tax
$
9.0

 
$
4.1

 
$
20.5

 
$
11.1

RSUs granted
91,579

 
31,720

 
513,408

 
236,919

Weighted average grant date fair value per unit
$
103.41

 
$
103.64

 
$
92.92

 
$
99.77

Total fair value of vested RSUs
$
3.5

 
$
2.4

 
$
22.3

 
$
26.2

The following table shows a summary of PSU activity during the three and nine months ended September 30, 2018 and 2017:
 
Three Months Ended 
 September 30,
 
Nine Months Ended 
 September 30,
 
2018
 
2017
 
2018
 
2017
PSU compensation expense
$
6.1

 
$
4.9

 
$
13.4

 
$
5.0

Income tax benefit
(0.8
)
 
(1.7
)
 
(2.1
)
 
(1.7
)
PSU compensation expense, net of tax
$
5.3

 
$
3.2

 
$
11.3

 
$
3.3

PSUs granted
164,957

 
336

 
164,957

 
237,959

Weighted average grant date fair value per unit
$
123.51

 
$
112.32

 
$
123.51

 
$
112.32

Total fair value of vested PSUs
$
1.9

 
$
0.3

 
$
27.5

 
$
29.7