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9. Share-based Compensation (Tables)
12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement, Noncash Expense [Abstract]  
Stock Options Activity

Stock Options
The following table summarizes stock option activity:

 
 
Shares
 
Weighted-
Average
Exercise Price
 
Weighted-
Average
Remaining
Contractual
Term (in years)
 
Aggregate
Intrinsic
Value
(in millions)
Outstanding, December 31, 2018
 
373,640

 
$
138.81

 
 
 
 
Granted
 
34,672

 
$
328.48

 
 
 
 
Exercised
 
(55,490
)
 
$
98.00

 
 
 
 
Forfeited/expired
 
(8,300
)
 
$
231.71

 
 
 
 
Outstanding, December 31, 2019
 
344,522

 
$
162.23

 
4.76
 
$
71.6

 
 
 
 
 
 
 
 
 
Unvested, December 31, 2019
 
94,172

 
$
273.93

 
8.45
 
$
9.1

Exercisable, December 31, 2019
 
250,350

 
$
120.22

 
3.46
 
$
62.5


Stock Options Valuation Assumptions

The weighted-average fair value of stock options granted was estimated using a Black-Scholes option-pricing model with the following weighted-average assumptions:

 
 
Year Ended December 31,
 
 
2019
 
2018
 
2017
Expected volatility
 
22
%
 
22
%
 
20
%
Risk-free interest rate
 
1.69
%
 
2.85
%
 
1.87
%
Expected life (in years)
 
7.5

 
7.6

 
7.2

Expected dividend
 

 

 

Weighted-average fair value of options granted
 
$
93.96

 
$
105.94

 
$
58.65


Resticted Stock Activity

The following table summarizes restricted stock unit activity:
 
 

Restricted Stock
Units
 
Weighted-
Average
Grant-Date
Fair Value
 
Weighted-Average
Remaining
Contractual Term
(in years)
 
Aggregate
Intrinsic Value
(in millions)
Outstanding, December 31, 2018
 
474,400

 
$
235.58

 
 
 
 
Granted
 
174,782

 
$
332.38

 
 
 
 
Vested
 
(134,525
)
 
$
206.44

 
 
 
 
Forfeited
 
(44,715
)

$
243.76

 
 
 
 
Outstanding, December 31, 2019
 
469,942

 
$
279.14

 
2.08
 
$
173.9


Employee Stock Purchase Plan, Valuation Assumptions
The fair value of the employees’ purchase rights under the 2011 ESPP was estimated using a Black-Scholes model with the following weighted-average assumptions:

 
Year Ended December 31,
 
2019
 
 
 
2018
 
 
 
2017
Expected volatility
31
%
 
 
 
27
%
 
 
 
19
%
Risk-free interest rate
2.25
%
 
 
 
1.82
%
 
 
 
0.83
%
Expected life (in years)
0.24

 
 
 
0.24

 
 
 
0.24

Expected dividend

 
 
 

 
 
 

Weighted-average fair value
 
 
 
 
 
 
 
 
 
of purchase rights
$
60.39

 
 
 
$
55.04

 
 
 
$
38.86