Stock Options (Schedule of Qualified Stock Option Activity) (Details) - Qualified Stock Options - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shares
Options outstanding at beginning of year (in shares)	119,720	140,170	172,875
Granted (in shares)	13,950	12,150	9,500
Exercised (in shares)	(31,300)	(28,100)	(41,205)
Canceled (in shares)	(1,450)	(4,500)	(1,000)
Options outstanding at end of year (in shares)	100,920	119,720	140,170
Exercise Price
Outstanding at beginning of year, exercise price (in dollars per share)	$ 57.65	$ 49.78	$ 43.91
Granted, exercise price (in dollars per share)	114.53	105.56	92.50
Exercised, exercise price (in dollars per share)	45.94	38.93	34.89
Canceled, exercise price (in dollars per share)	96.97	58.57	54.49
Outstanding at end of year, exercise price (in dollars per share)	$ 68.58	$ 57.65	$ 49.78
Options exercisable at end of year (in shares)	67,700	85,070	96,320
Exercisable at end of year, exercise price (in dollars per share)	$ 52.60	$ 47.10	$ 41.56
Options available for grant at end of year (in shares)	314,750	327,250	336,450