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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Pension Benefit Plan [Member]      
Periodic costs for employee retirement benefit plans      
Service Cost $ 2,426 $ 2,399 $ 2,308
Interest Cost 3,061 3,143 3,046
Expected Return on Plan Assets (4,871) (4,489) (4,014)
Amortization of Net Actuarial Loss 1,658 1,566 1,426
Amortization of Prior Service Credit
Net Periodic Benefit Cost [1] 2,274 2,619 2,766
Other Benefits Plan [Member]      
Periodic costs for employee retirement benefit plans      
Service Cost 1,135 1,089 1,099
Interest Cost 1,898 1,964 1,952
Expected Return on Plan Assets (2,550) (2,406) (2,213)
Amortization of Net Actuarial Loss 1,787 1,781 1,773
Amortization of Prior Service Credit (1,607) (1,728) (1,728)
Net Periodic Benefit Cost [1] $ 663 $ 700 $ 883
[1] Service cost is included in Operations and Maintenance expense on the consolidated statements of income; all other amounts are included in Other Income (Expense), net.