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Share-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Recent Share-Based Compensation Expense
The following is a summary of share-based compensation expense for the years ended December 31, 2015, 2016 and 2017:
Year
 
Share-Based
Compensation
Expense
 
Tax Benefit
2015
 
$
34.2

 
$
13.2

2016
 
39.2

 
15.1

2017
 
40.4

 
13.6
Schedule of Transactions of the Company's Stock Options
The following table summarizes the transactions of the Company’s stock options:
 
Stock Options
 
Weighted
Average
Exercise
Price
 
Weighted
Average
Remaining
Contractual
Life (years)
Unexercised options outstanding—January 1, 2017
1.4

 
$
108.53

 
 
Options granted
0.0

 
168.60

 
 
Options exercised
(0.7
)
 
97.54

 
 
Options forfeited
(0.1
)
 
140.26

 
 
Unexercised options outstanding—December 31, 2017
0.6

 
122.04

 
3.9
Exercisable at December 31, 2017
0.2

 
115.74

 
1.7
Schedule of Assumptions Used to Determine Fair Value of Options Granted
The fair value of options granted was estimated using the Black-Scholes option pricing model. The weighted average fair value of options granted during the years ended December 31, 2015, 2016 and 2017 was $54.92, $39.02 and $48.05, per option, respectively, based on the weighted-average grant date assumptions stated below.
 
For the Years Ended December 31,
 
2015
 
2016
 
2017
Dividend yield
0.0
%
 
0.0
%
 
0.5
%
Expected volatility(1)
26.7
%
 
30.7
%
 
28.0
%
Risk-free interest rate(2)
1.5
%
 
1.6
%
 
2.1
%
Expected life of options (in years)(3)
5.0

 
5.7

 
5.7

Forfeiture rate
0.0
%
 
0.0
%
 
0.0
%
__________________________

(1) 
Expected volatility is based on historical and implied volatility.

(2) 
Risk-free interest rate is based on the U.S. Treasury yield curve in effect at the date of grant.

(3) 
Expected life of options (in years) is based on the Company’s historical and expected exercise behavior.
Schedule of Transactions of the Company's Restricted Stock
The following table summarizes the transactions of the Company’s restricted stock:
 
Restricted
Stock
 
Weighted
Average
Grant Date
Value
Unvested units—January 1, 2017
0.6

 
$
168.84

Units granted
0.2

 
152.99

Units vested
(0.4
)
 
166.22

Units forfeited
(0.0
)
 
170.39

Unvested units—December 31, 2017
0.4

 
162.32