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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Number of shares      
Shares/Share units potentially payable, beginning balance 112,465 157,405 189,418
Forfeitures
Payments (45,689) (79,762) (75,545)
Shares accrued 14,936 34,822 43,532
Shares/Share units potentially payable, ending balance 81,712 112,465 157,405
Weighted average grant date value per share      
Shares/Share units potentially payable, beginning balance $ 49.96 $ 40.30 $ 36.90
Forfeitures  
Payments 36.74 39.90 36.35
Shares accrued 92.12 70.59 48.26
Shares/Share units potentially payable, ending balance $ 65.06 $ 49.96 $ 40.30
Year-end intrinsic value      
Shares potentially payable $ 5,619 $ 6,343 $ 6,989
Shares potentially payable $ 5,316 $ 5,619 $ 6,343