Share-Based Compensation Plans (Schedule of Option Activity) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at the beginning of year (in shares)	259,119
Granted (in shares)	0	0
Exercised (in shares)	(152,634)
Forfeited or expired (in shares)	0
Outstanding at the end of year (in shares)	106,485
Exercisable at the end of year (in shares)	105,939
Outstanding at the beginning of year, Weighted-Average Exercise Price (in dollars per share)	$ 56.66
Granted, Weighted-Average Exercise Price (in dollars per share)	0.00
Exercised, Weighted-Average Exercise Price (in dollars per share)	65.37
Forfeited or Expired, Weighted-Average Exercise Price (in dollars per share)	0.00
Outstanding at the end of year, Weighted-Average Exercise Price (in dollars per share)	44.18
Exercisable at the end of the year, Weighted-Average Exercise Price (in dollars per share)	$ 43.62
Outstanding at the end of year, Weighted-Average Remaining Contractual Term (in years)	1 year 2 months
Exercisable at the end of year, Weighted-Average Remaining Contractual Term (in years)	1 year 1 month
Outstanding at the end of year, Aggregate Intrinsic Value	$ 9,451
Exercisable at the end of year, Aggregate Intrinsic Value	$ 9,451