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Stock-Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum shares available for grants 8,000,000    
Share-based compensation expense $ 19 $ 19 $ 20
Employee service share-based compensation, tax benefit from compensation expense 7 7 8
Amounts paid to settle share units 27 $ 33 $ 11
Compensation cost not yet recognized $ 21    
Expected weighted average recognition period for share-based compensation expense, in months 23 months    
Performance Share Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance period 3 years    
Percentage of shares issued per share unit, minimum 0.00%    
Percentage of shares issued per share unit, maximum 200.00%    
Fair value of each share unit, per share $ 52.88 [1] $ 38.90  
Closing common share price   $ 46.13 $ 36.16
Three-year risk-free rate 1.10% 0.78%  
Volatility rate, minimum 12.00% 12.00%  
Volatility rate, maximum 18.00% 18.00%  
[1] Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2015 under the 2014 Incentive Plan.