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9. Share-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2016
Share-based Compensation [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, January 1, 2014
 
629,643

 
$
78.72

 
 
 
 
Granted
 
54,000

 
$
119.71

 
 
 
 
Exercised
 
(91,387
)
 
$
63.66

 
 
 
 
Forfeited/expired
 
(10,450
)
 
$
100.29

 
 
 
 
Outstanding, December 31, 2014
 
581,806

 
$
84.50

 
 
 
 
Granted
 
40,500

 
$
139.56

 
 
 
 
Exercised
 
(195,221
)
 
$
61.29

 
 
 
 
Forfeited/expired
 
(380
)
 
$
62.47

 
 
 
 
Outstanding, December 31, 2015
 
426,705

 
$
100.36

 
 
 
 
Granted
 
45,000

 
$
159.37

 
 
 
 
Exercised
 
(15,285
)
 
$
80.70

 
 
 
 
Forfeited/expired
 
(6,850
)
 
$
127.65

 
 
 
 
Outstanding, December 31, 2016
 
449,570

 
$
106.52

 
5.15
 
$
34.1

Vested and expected to vest,
 
 
 
 
 
 
 
 
December 31, 2016
 
437,350

 
$
105.39

 
5.04
 
$
33.6

Exercisable, December 31, 2016
 
314,330

 
$
93.78

 
3.80
 
$
27.8
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,
 
 
2016
 
2015
 
2014
Expected volatility
 
21
%
 
23
%
 
25
%
Risk-free interest rate
 
1.35
%
 
1.90
%
 
2.35
%
Expected life (in years)
 
7.4

 
7.7

 
8.7

Expected dividend
 
—

 
—

 
—

Weighted-average fair value of options granted
 
$
42.40

 
$
42.74

 
$
43.96
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, January 1, 2014
 
375,430

 
$
105.44

 
 
 
 
Granted
 
145,695

 
$
119.56

 
 
 
 
Vested
 
(107,557
)
 
$
98.52

 
 
 
 
Forfeited
 
(36,203
)
 
$
108.17

 
 
 
 
Outstanding, December 31, 2014
 
377,365

 
$
112.60

 
 
 
 
Granted
 
177,110

 
$
139.56

 
 
 
 
Vested
 
(113,347
)
 
$
107.62

 
 
 
 
Forfeited
 
(28,706
)

$
116.19

 
 
 
 
Outstanding, December 31, 2015
 
412,422

 
$
125.30

 
 
 
 
Granted
 
193,025

 
$
159.32

 
 
 
 
Vested
 
(117,203
)
 
$
119.33

 
 
 
 
Forfeited
 
(39,422
)

$
129.80

 
 
 
 
Outstanding, December 31, 2016
 
448,822

 
$
141.09

 
2.16
 
$
81.8
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,
 
2016
 
 
 
2015
 
 
 
2014
Expected volatility
20
%
 
 
 
18
%
 
 
 
15
%
Risk-free interest rate
0.26
%
 
 
 
0.02
%
 
 
 
0.04
%
Expected life (in years)
0.25

 
 
 
0.25

 
 
 
0.25

Expected dividend
—

 
 
 
—

 
 
 
—

Weighted-average fair value
 
 
 
 
 
 
 
 
 
of purchase rights
$
27.36

 
 
 
$
25.08

 
 
 
$
21.88