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Stock-Based Compensation (Narrative) (Details)
$ / shares in Units, $ in Millions
6 Months Ended
Jun. 30, 2017
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Unrecognized compensation cost | $ $ 39
Cost expected to be recognized, weighted average period (in years) 2 years
Market Based Restricted Stock [Member] [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Award term cliff vest at the end of three years
Vesting period (in years) for shares awarded 3 years
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%
Performance-Based Restricted Stock [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Award term cliff vest at the end of three years
Number of performance-based shares awarded to certain senior executives | shares 149,843
Vesting period (in years) for shares awarded 3 years
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%
Weighted average grant-date fair value (in dollars per share) | $ / shares $ 51.90
Method used for calculation of performance of share Monte Carlo
Weighted average stock price volatility 21.10%
Expected term (in years) 3 years
Risk free interest rate 1.43%
Time Vested Stock [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of performance-based shares awarded to certain senior executives | shares 144,141
Weighted average grant-date fair value (in dollars per share) | $ / shares $ 55.55
Restricted Stock [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Aggregate market value of shares released and issued | $ $ 26
Minimum [Member] | Time Vested Stock [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Vesting period (in years) for shares awarded 3 years
Maximum [Member] | Time Vested Stock [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Vesting period (in years) for shares awarded 5 years