Stock Award Plans, Director Plan Outstanding and Exercisable Options (Details) - Director Plan [Member] - Stock Options [Member]	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
First Price Range [Member]
Outstanding and Exercisable Stock options [Abstract]
Range of Exercise Prices, upper limit (in dollars per share)	$ 29.60
Options Outstanding [Abstract]
Number Outstanding (in shares) | shares	6,000
Weighted Average Remaining Contractual Life	1 year 3 months 18 days
Weighted Average Exercise Price, Options Outstanding (in dollars per share)	$ 29.60
Options Exercisable [Abstract]
Number Exercisable (in shares) | shares	6,000
Weighted Average Exercise Price, Options Exercisable (in dollars per share)	$ 29.60
Second Price Range [Member]
Outstanding and Exercisable Stock options [Abstract]
Range of Exercise Prices, lower limit (in dollars per share)	41.24
Range of Exercise Prices, upper limit (in dollars per share)	$ 56.45
Options Outstanding [Abstract]
Number Outstanding (in shares) | shares	53,276
Weighted Average Remaining Contractual Life	2 years 4 months 24 days
Weighted Average Exercise Price, Options Outstanding (in dollars per share)	$ 52.77
Options Exercisable [Abstract]
Number Exercisable (in shares) | shares	53,276
Weighted Average Exercise Price, Options Exercisable (in dollars per share)	$ 52.77
Third Price Range [Member]
Outstanding and Exercisable Stock options [Abstract]
Range of Exercise Prices, lower limit (in dollars per share)	61.89
Range of Exercise Prices, upper limit (in dollars per share)	$ 62.48
Options Outstanding [Abstract]
Number Outstanding (in shares) | shares	35,153
Weighted Average Remaining Contractual Life	4 years 6 months
Weighted Average Exercise Price, Options Outstanding (in dollars per share)	$ 62.31
Options Exercisable [Abstract]
Number Exercisable (in shares) | shares	35,153
Weighted Average Exercise Price, Options Exercisable (in dollars per share)	$ 62.31
Fourth Price Range [Member]
Outstanding and Exercisable Stock options [Abstract]
Range of Exercise Prices, lower limit (in dollars per share)	70.65
Range of Exercise Prices, upper limit (in dollars per share)	$ 99.52
Options Outstanding [Abstract]
Number Outstanding (in shares) | shares	63,188
Weighted Average Remaining Contractual Life	6 years
Weighted Average Exercise Price, Options Outstanding (in dollars per share)	$ 86.50
Options Exercisable [Abstract]
Number Exercisable (in shares) | shares	62,391
Weighted Average Exercise Price, Options Exercisable (in dollars per share)	$ 86.71
Full Exercise Price Range [Member]
Outstanding and Exercisable Stock options [Abstract]
Range of Exercise Prices, lower limit (in dollars per share)	29.60
Range of Exercise Prices, upper limit (in dollars per share)	$ 99.52
Options Outstanding [Abstract]
Number Outstanding (in shares) | shares	157,617
Weighted Average Remaining Contractual Life	4 years 3 months 18 days
Weighted Average Exercise Price, Options Outstanding (in dollars per share)	$ 67.54
Aggregate Intrinsic Value, Options Outstanding | $	$ 1,128,000
Options Exercisable [Abstract]
Number Exercisable (in shares) | shares	156,820
Weighted Average Exercise Price, Options Exercisable (in dollars per share)	$ 67.52
Aggregate Intrinsic Value, Options Exercisable | $	$ 1,128,000