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Stock Based Incentive Compensation Plans (Summary of activity for stock options with performance conditions) (Details) - Stock options with performance conditions [Member]
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Summary of activity for stock options [Roll Forward]  
Outstanding as of beginning of year (in shares) | shares 779,816
Granted - original (in shares) | shares 102,728
Exercised (includes shares tendered) (in shares) | shares (596,374)
Forfeited or expired (in shares) | shares (14,446)
Outstanding as of end of year (in shares) | shares 271,724
Vested or expected to vest as of end of year (in shares) | shares 236,907 [1]
Exercisable as of end of year (in shares) | shares 202,090
Summary of activity for stock options, additional disclosures [Abstract]  
Weighted average exercise price as of beginning of year (in dollars per share) | $ / shares $ 49.83
Weighted average exercise price granted - original (in dollars per share) | $ / shares 69.01
Weighted average exercise price exercised (includes shares tendered) (in dollars per share) | $ / shares 51.13
Weighted average exercise price forfeited or expired (in dollars per share) | $ / shares 47.44
Weighted average exercise price outstanding as of end of year (in dollars per share) | $ / shares 54.37
Weighted average exercise price vested or expected to vest as of end of year (in dollars per share) | $ / shares 53.68 [1]
Weighted average exercise price exercisable as of end of year (in dollars per share) | $ / shares $ 52.75
Weighted average remaining contractual term outstanding as of end of year (in years) 2 years 8 months 9 days
Weighted average remaining contractual term vested or expected to vest as of end of year (in years) 2 years 6 months 29 days [1]
Weighted average remaining contractual term exercisable as of end of year (in years) 2 years 5 months 5 days
Aggregate intrinsic value outstanding as of end of year | $ $ 6
Aggregate intrinsic value vested or expected to vest as of end of year | $ 5 [1]
Aggregate intrinsic value exercisable as of end of year | $ $ 5
[1] Includes estimated forfeitures.