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Stock-based compensation (Tables)
 12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Components of stock-based compensation expense recognized in the Consolidated Statements of Income
The following table reflects the components of stock-based compensation expense recognized in our Consolidated Statements of Income (in millions):
 
Years ended December 31,
 
2018
 
2017
 
2016
RSUs
$
165

 
$
174

 
$
177

Performance units
117

 
133

 
123

Stock options
29

 
22

 
11

Total stock-based compensation expense, pretax
311

 
329

 
311

Tax benefit from stock-based compensation expense
(67
)
 
(118
)
 
(112
)
Total stock-based compensation expense, net of tax
$
244

 
$
211

 
$
199
Summary of RSUs
The following table summarizes information regarding our RSUs:
 
Year ended December 31, 2018
 
Units
(in millions)
 
Weighted-average
grant date
fair value
Balance nonvested at December 31, 2017
3.4

 
$
155.11

Granted
1.2

 
$
179.18

Vested
(1.2
)
 
$
142.82

Forfeited
(0.3
)
 
$
163.36

Balance nonvested at December 31, 2018
3.1

 
$
168.11
Weighted-average assumptions used and the resulting weighted-average grant date fair value of stock options
The weighted-average assumptions used in the option valuation model and the resulting weighted-average grant date fair values of stock options granted were as follows:
 
Years ended December 31,
 
2018
 
2017
 
2016
Closing price of our common stock on grant date
$
177.46

 
$
162.60

 
$
156.35

Expected volatility (average of implied and historical volatility)
24.6
%
 
22.7
%
 
24.3
%
Expected life (in years)
5.8

 
5.8

 
5.8

Risk-free interest rate
2.8
%
 
2.1
%
 
1.5
%
Expected dividend yield
2.9
%
 
2.8
%
 
2.6
%
Fair value of stock options granted
$
34.60

 
$
27.54

 
$
27.55
Summary of stock options
The following table summarizes information regarding our stock options:
 
Year ended December 31, 2018
 
Options
(in millions)
 
Weighted-
average
exercise price
 
Weighted-
average
remaining
contractual
life (in years)
 
Aggregate
intrinsic
value
(in millions)
Balance unexercised at December 31, 2017
4.0

 
$
127.08

 
 
 
 
Granted
1.2

 
$
177.46

 
 
 
 
Exercised
(0.5
)
 
$
76.68

 
 
 
 
Expired/forfeited
(0.3
)
 
$
163.79

 
 
 
 
Balance unexercised at December 31, 2018
4.4

 
$
143.57

 
7.0
 
$
225

Vested or expected to vest at December 31, 2018
4.2

 
$
142.09

 
6.9
 
$
220

Exercisable at December 31, 2018
1.2

 
$
84.01

 
3.3
 
$
135
Weighted average assumptions used and the resulting weighted average grant date fair value of performance units
The weighted-average assumptions used in this model and the resulting weighted-average grant date fair values of performance units granted were as follows:
 
Years ended December 31,
 
2018
 
2017
 
2016
Closing price of our common stock on grant date
$
177.93

 
$
162.60

 
$
156.35

Volatility
23.8
%
 
25.9
%
 
25.8
%
Risk-free interest rate
2.6
%
 
1.4
%
 
0.9
%
Fair value of units granted
$
189.21

 
$
178.87

 
$
170.56