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Stock Options and Incentive Plans (Schedules of Other Share-based Compensation Activity) (Details) - Restricted Stock [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 261,145 252,866 347,941
Grants 96,505 118,279 90,065
Changes due to performance (11,891) 18,779 13,966
Payments (89,190) (88,073) (167,482)
Forfeitures (20,473) (40,706) (31,624)
Shares/Share units potentially payable, ending balance 236,096 261,145 252,866
Weighted average grant date value per share      
Payments $ 46.64 $ 33.20 $ 36.08
Cash paid for share based liabilities $ 4,160 $ 2,924 $ 6,040