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Stock-Based Incentive Compensation Plans (Summary of activity for stock appreciation rights plan) (Details) - Non-employee Stock Appreciation Rights [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 237,272
Granted - original (in shares) | shares 26,494
Exercised (includes shares tendered) (in shares) | shares (93,159)
Forfeited or expired (in shares) | shares (8,419)
Outstanding as of end of year (in shares) | shares 162,188
Vested or expected to vest as of end of year (in shares) | shares 155,300 [1]
Exercisable as of end of year (in shares) | shares 92,092
Summary of activity for stock options, additional disclosures [Abstract]  
Weighted average exercise price as of beginning of year (in dollars per share) | $ / shares $ 42.59
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 37.02
Weighted average exercise price forfeited or expired (in dollars per share) | $ / shares 40.40
Weighted average exercise price outstanding as of end of year (in dollars per share) | $ / shares 50.22
Weighted average exercise price vested or expected to vest as of end of year (in dollars per share) | $ / shares 50.18 [1]
Weighted average exercise price exercisable as of end of year (in dollars per share) | $ / shares $ 48.72
Weighted average remaining contractual term outstanding as of end of year (in years) 2 years 6 months
Weighted average remaining contractual term vested or expected to vest as of end of year (in years) 2 years 5 months 12 days [1]
Weighted average remaining contractual term exercisable as of end of year (in years) 1 year 10 months 17 days
Aggregate intrinsic value outstanding as of end of year | $ $ 4
Aggregate intrinsic value vested or expected to vest as of end of year | $ 4 [1]
Aggregate intrinsic value exercisable as of end of year | $ $ 3
[1] Includes estimated forfeitures.