Stock Options (Schedule of Non-Qualified Stock Options Activity) (Details) - Non Qualified Options - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding at beginning of year (in shares)	112,400	127,300	132,100
Granted (in shares)	0	0	0
Exercised (in shares)	(45,400)	(14,900)	(4,800)
Canceled (in shares)	0	0	0
Options outstanding at end of year (in shares)	67,000	112,400	127,300
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding at beginning of year, exercise price (in dollars per share)	$ 34.48	$ 32.05	$ 31.30
Granted, exercise price (in dollars per share)	0.00	0.00	0.00
Exercised, exercise price (in dollars per share)	22.73	13.73	11.45
Canceled, exercise price (in dollars per share)	0.00	0.00	0.00
Outstanding at end of year, exercise price (in dollars per share)	$ 34.48	$ 32.05	$ 31.30	$ 42.43	$ 34.48	$ 32.05
Options exercisable at end of year (in shares)				50,400	85,000	83,100
Exercisable at end of year, exercise price (in dollars per share)				$ 39.86	$ 29.61	$ 24.54
Options available for grant at end of year				176,294	207,608	232,522