Stock Award Plans, Director Plan Stock Option Activity (Details) - Director Plan [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Award Plan Information [Abstract]
Shares available for future grants (in shares)	510,071
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)	205,429	220,429	298,334
Granted (in shares)	3,188
Exercised (in shares)	(39,000)	(9,000)	(77,905)
Canceled or expired (in shares)	(12,000)	(6,000)
Outstanding at Period End (in shares)	157,617	205,429	220,429
Weighted Average Exercise Price [Abstract]
Outstanding at Period Start (in dollars per share)	$ 64.60	$ 64.37	$ 60.01
Granted (in dollars per share)	70.65
Exercised (in dollars per share)	46.23	35.72	47.65
Canceled or expired (in dollars per share)	87.35	99.52
Outstanding at Period End (in dollars per share)	$ 67.54	$ 64.60	$ 64.37
Stock options exercisable (in shares)	156,820	205,429	220,429
Restricted Stock [Member]
Stock Award Plan Information [Abstract]
Vesting period	6 months