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Employee Benefit Plans (Schedule of Periodic Costs For Employee Retirement Benefit Plans) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Benefit Plan [Member]      
Periodic costs for employee retirement benefit plans      
Service Cost $ 2,399 $ 2,308 $ 2,558
Interest Cost 3,143 3,046 2,894
Expected Return on Plan Assets (4,489) (4,014) (3,919)
Amortization of Net Actuarial Loss 1,566 1,426 1,645
Amortization of Prior Service Credit
Net Periodic Benefit Cost 2,619 2,766 3,178
Other Benefits Plan [Member]      
Periodic costs for employee retirement benefit plans      
Service Cost 1,089 1,099 1,373
Interest Cost 1,964 1,952 1,921
Expected Return on Plan Assets (2,406) (2,213) (2,107)
Amortization of Net Actuarial Loss 1,781 1,773 2,261
Amortization of Prior Service Credit (1,728) (1,728) (1,728)
Net Periodic Benefit Cost $ 700 $ 883 $ 1,720