Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 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 months ended March 31, 2016 and 2015 were as follows:
 
Three Months Ended
March 31
($ in millions)
2016
 
2015
Stock-based compensation
$
10

 
$
15

Income tax benefit recognized
$
4

 
$
5

Grants of stock-based compensation during the three months ended March 31, 2016 and 2015 were as follows:
 
 
Three Months Ended
March 31
 
 
2016
 
2015
Grant Details
 
Shares
 
Fair Value
 
Shares
 
Fair Value
Stock options
 
775,319

 
$
17.89

 
583,172

 
$
26.98

Restricted stock units
 
245,038

 
$
90.79

 
182,150

 
$
114.04

Contingent shares (a)
 
59,645

 
$
95.00

 
63,308

 
$
118.12

(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 three months ended March 31, 2016 was calculated with the following weighted average assumptions:
Weighted average exercise price
$
95.06

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

Expected dividend yield
2.1
%
Expected volatility
22.8
%