Stock Award Plans, Director Plan Stock Option Activity (Details) - Director Plan [Member] - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Award Plan Information [Abstract]
Shares available for future grants (in shares)	486,058
Stock Options [Member]
Stock Award Plan Information [Abstract]
Term of grant	10 years
Outstanding Non-Qualified or Nonincentive Stock Awards [Roll Forward]
Outstanding at Period Start (in shares)	157,617	205,429	220,429
Granted (in shares)	2,640	3,188
Exercised (in shares)	(29,153)	(39,000)	(9,000)
Forfeited (in shares)		(12,000)	(6,000)
Outstanding at Period End (in shares)	131,104	157,617	205,429
Weighted Average Exercise Price [Abstract]
Outstanding at Period Start (in dollars per share)	$ 67.54	$ 64.60	$ 64.37
Granted (in dollars per share)	85.30	70.65
Exercised (in dollars per share)	57.47	46.23	35.72
Forfeited (in dollars per share)		87.35	99.52
Outstanding at Period End (in dollars per share)	$ 70.14	$ 67.54	$ 64.60
Stock options exercisable (in shares)	130,444	156,820	205,429
Restricted Stock [Member]
Stock Award Plan Information [Abstract]
Vesting period	6 months