Stock Options and Incentive Plans (Schedules of Executive Management Share-based Compensation Activity) (Details) - Performance Shares [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of shares
Shares/Share units potentially payable, beginning balance	189,418	188,308	185,199
Forfeitures
Payments	(75,545)	(86,926)	(95,889)
Shares accrued	43,532	88,036	98,998
Shares/Share units potentially payable, ending balance	157,405	189,418	188,308
Weighted average grant date value per share
Shares/Share units potentially payable, beginning balance	$ 36.90	$ 35.35	$ 30.69
Forfeitures
Payments	36.35	33.43	29.09
Shares accrued	48.26	36.78	38.01
Shares/Share units potentially payable, ending balance	$ 40.30	$ 36.90	$ 35.35
Year-end intrinsic value
Shares potentially payable	$ 6,989	$ 6,657	$ 5,683
Shares potentially payable	$ 6,343	$ 6,989	$ 6,657