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Stock-Based Awards and Other Equity Instruments (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Option Activity
The following table presents a summary of our stock option activity:
 
Options
 
Weighted Average
Exercise Price
 
Remaining
Contractual Life
 
Aggregate
Intrinsic Value
 
(In thousands)
 
 
 
(In years)
 
(In thousands)
Balance as of January 1, 2015
14,435

 
$
49.33

 
 
 
 
Granted
7,572

 
94.13

 
 
 
 
Exercised
(4,201
)
 
34.57

 
 
 
 
Cancelled
(751
)
 
74.06

 
 
 
 
Balance as of December 31, 2015
17,055

 
71.77

 
 
 
 
Granted
5,670

 
105.37

 
 
 
 
Exercised
(2,686
)
 
47.57

 
 
 
 
Cancelled
(1,198
)
 
91.62

 
 
 
 
Balance as of December 31, 2016
18,841

 
84.07

 
 
 
 
Granted
3,618

 
124.08

 
 
 
 
Exercised
(3,422
)
 
62.67

 
 
 
 
Cancelled
(3,384
)
 
96.86

 
 
 
 
Balance as of December 31, 2017
15,653

 
95.23

 
4.4
 
$
402,509

Exercisable as of December 31, 2017
5,903

 
74.64

 
3.1
 
266,849

Vested and expected to vest after December 31, 2017
15,653

 
95.23

 
4.4
 
402,509
Weighted Average Assumptions of Black-Scholes and Monte Carlo Option-Pricing Models
The fair value of stock options granted during the years ended December 31, 2017, 2016 and 2015 were estimated at the date of grant using appropriate valuation techniques, including the Black-Scholes and Monte Carlo option-pricing models, assuming the following weighted average assumptions:
 
2017
 
2016
 
2015
Risk-free interest rate
1.58
%
 
0.97
%
 
1.19
%
Expected volatility
32.47
%
 
39.06
%
 
41.48
%
Expected life (in years)
3.65

 
3.59

 
4.06

Dividend yield
0.92
%
 
0.91
%
 
0.78
%
Weighted-average estimated fair value of options granted during the year
$
30.17

 
$
29.48

 
$
30.56
Summary of Restricted Stock Units Activity
The following table presents a summary of RSU activity:
 
RSUs
 
Weighted Average
Grant-Date Fair
Value
 
(In thousands)
 
 
Balance as of January 1, 2015
337

 
$
61.97

Granted
1,643

 
123.42

Vested
(493
)
 
103.73

Cancelled
(91
)
 
67.11

Balance as of December 31, 2015
1,396

 
119.20

Granted
691

 
109.38

Vested
(516
)
 
118.71

Cancelled
(222
)
 
117.84

Balance as of December 31, 2016
1,349

 
114.58

Granted
1,350

 
123.24

Vested
(492
)
 
115.29

Cancelled
(266
)
 
116.26

Balance as of December 31, 2017
1,941

 
120.19