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Pension Plans And Other Postretirement Benefit Plans (Change in Benefit Obligation and Plan Assets) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax $ (8,090) $ (7,568)  
Pension Plan And SERP [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation as of beginning of year 666,472 613,503  
Service cost 20,406 18,302 $ 19,791
Interest cost 27,898 27,544 26,117
Actuarial (gain)/loss 39,743 39,997  
Plan Change 3,158 0  
Cumulative adjustment to reclassify liability 0 0  
Benefits paid (41,116) (32,874)  
Benefit obligation as of end of year 716,561 666,472 613,503
Fair value of plan assets as of beginning of year 540,914 517,234  
Actual return on plan assets 82,476 43,212  
Employer contributions 22,000 12,000  
Fair value of plan assets as of end of year 605,652 540,914 517,234
Funded status (110,909) (125,558)  
Unrecognized net actuarial loss 157,883 178,783  
Unrecognized prior service cost 3,179 23  
Prepaid (accrued) benefit cost 50,153 53,248  
Additional liability (161,062) (178,806)  
Accrued benefit liability (110,909) (125,558)  
Accumulated pension benefit obligation 624,345 583,498  
Unrecognized prior service cost 2,066 15  
Unrecognized net actuarial loss 102,624 116,209  
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Before Regulatory Asset, Net of Tax 104,690 116,224  
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax 7,665 7,321  
Less regulatory asset $ (97,025) $ (108,903)  
Discount rate for benefit obligation 3.71% 4.26%  
Discount rate for annual expense 4.26% 4.57%  
Expected long-term return on plan assets 5.87% 5.40%  
Rate of compensation increase 4.69% 4.78%  
Expected return on plan assets $ (31,626) $ (27,547) (28,299)
Defined Benefit Plans Benefits Paid (39,738) (31,532)  
Other Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation as of beginning of year 136,453 138,795  
Service cost 3,220 3,205 2,925
Interest cost 5,490 6,110 5,158
Actuarial (gain)/loss (6,020) (3,648)  
Plan Change 0 0  
Cumulative adjustment to reclassify liability 0 (1,042)  
Benefits paid (6,196) (6,967)  
Benefit obligation as of end of year 132,947 136,453 138,795
Fair value of plan assets as of beginning of year 33,365 30,868  
Actual return on plan assets 4,588 2,497  
Employer contributions 0 0  
Fair value of plan assets as of end of year 37,953 33,365 30,868
Funded status (94,994) (103,088)  
Unrecognized net actuarial loss 68,280 81,979  
Unrecognized prior service cost (7,782) (8,981)  
Prepaid (accrued) benefit cost (34,496) (30,090)  
Additional liability (60,498) (72,998)  
Accrued benefit liability (94,994) (103,088)  
Accumulated pension benefit obligation 0 0  
Unrecognized prior service cost (5,058) (5,854)  
Unrecognized net actuarial loss 44,382 53,303  
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Before Regulatory Asset, Net of Tax 39,324 47,449  
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax 425 247  
Less regulatory asset $ (38,899) $ (47,202)  
Discount rate for benefit obligation 3.72% 4.23%  
Discount rate for annual expense 4.23% 4.57%  
Expected long-term return on plan assets 5.69% 6.03%  
Expected return on plan assets $ (1,899) $ (1,861) $ (1,991)
Defined Benefit Plans Benefits Paid $ 0 $ 0  
Other Postretirement Benefits [Member] | Pre-Age 65 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Medical cost trend - initial 6.50% 7.00%  
Medical cost trend - ultimate 5.00% 5.00%  
Ultimate medical cost trend year 2023 2023  
Other Postretirement Benefits [Member] | Post-Age 65 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Medical cost trend - initial 6.50% 7.00%  
Medical cost trend - ultimate 5.00% 5.00%  
Ultimate medical cost trend year 2024 2024  
Retirees [Member] | Other Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation as of beginning of year $ 60,670    
Benefit obligation as of end of year 60,354 $ 60,670  
Fully eligible employees [Member] | Other Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation as of beginning of year 34,429    
Benefit obligation as of end of year 32,891 34,429  
Other participants [Member] | Other Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation as of beginning of year 41,354    
Benefit obligation as of end of year $ 39,702 $ 41,354