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Stock-Based Compensation (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
9 Months Ended
Sep. 30, 2012
Performance-Based-Achieved [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Estimated fair value for the shares awarded (in dollars per share) $ 33.98
Percentage of shares awarded on market performance target (as a percent) 100.00%
Percentage of shares awarded on market performance in excess of target (as a percent) 49.00%
Percentage of stock awarded based on market performance achieved (as a percent) 149.00%
Performance-Based Restricted Stock [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Level of market performance achieved based on shares awarded, minimum (as a percent) 0.00%
Level of market performance achieved based on shares awarded, maximum (as a percent) 200.00%
Method used for calculation of performance of share The fair value of the performance-based shares awarded was calculated using a Monte Carlo valuation model.
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 216,188
Estimated fair value for the shares awarded (in dollars per share) $ 39.52
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Weighted Average Volatility Rate 27.80%
Expected term (in years) 3
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate 0.36%
Time-Vested Restricted Stock [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 126,582
Estimated fair value for the shares awarded (in dollars per share) $ 33.75
Restricted Stock [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Unrecognized compensation cost $ 8
Cost expected to be recognized, weighted average period (in years) 1.4
Aggregate market value of shares released and issued $ 19
Restricted Stock [Member] | Performance-Based Restricted Stock [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Vesting period (in years) for shares awarded 3 years
Restricted Stock [Member] | Time-Vested Restricted Stock [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Vesting period (in years) for shares awarded 3 years