Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Details of Grants of Stock-based Compensation
Stock-based compensation and the income tax benefit recognized during the three and nine months ended September 30, 2016 and 2015 were as follows:
 
Three Months Ended
September 30
 
Nine Months Ended
September 30
($ in millions)
2016
 
2015
 
2016
 
2015
Stock-based compensation
$
12

 
$
2

 
$
38

 
$
42

Income tax benefit recognized
$
3

 
$
1

 
$
13

 
$
14

Grants of stock-based compensation during the nine months ended September 30, 2016 and 2015 were as follows:
 
Nine Months Ended
September 30
 
2016
 
2015
Grant Details
Shares
 
Fair Value
 
Shares
 
Fair Value
Stock options
705,334

 
$
17.94

 
569,650

 
$
26.94

Restricted stock units
251,824

 
$
91.55

 
190,921

 
$
113.75

Contingent shares (a)
62,116

 
$
95.46

 
63,062

 
$
117.91

(a) The number of contingent shares represents the target value of the award.
Weighted Average Assumptions Used in Calculating the Fair Value of Stock Option
The fair value of the stock option grants issued in the nine months ended September 30, 2016 was calculated with the following weighted average assumptions:
Weighted average exercise price
$
95.29

Risk-free interest rate
1.6
%
Expected life of option in years
6.5

Expected dividend yield
2.1
%
Expected volatility
22.8
%