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Employee Benefit Plans (Schedule of Benefits Plans) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Pension Benefit Plan [Member]        
Periodic costs for employee retirement benefit plans        
Service Cost $ 607 $ 600 $ 1,820 $ 1,799
Interest Cost 765 786 2,296 2,357
Expected Return on Assets (1,218) (1,122) (3,653) (3,367)
Amortization of Unrecognized Losses 415 391 1,244 1,174
Amortization of Unrecognized Prior Service Cost (Credit)
Net Periodic Benefit Cost [1] 569 655 1,707 1,963
Other Benefits Plan [Member]        
Periodic costs for employee retirement benefit plans        
Service Cost 284 272 851 817
Interest Cost 474 491 1,423 1,473
Expected Return on Assets (637) (601) (1,912) (1,804)
Amortization of Unrecognized Losses 447 445 1,340 1,336
Amortization of Unrecognized Prior Service Cost (Credit) (402) (432) (1,205) (1,296)
Net Periodic Benefit Cost [1] $ 166 $ 175 $ 497 $ 526
[1] Service cost is included in Operations and Maintenance expense on Consolidated Statements of Income; all other amounts are included in Other Income/Expense, net.