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Benefit Plans Reconciliation of Funded Status (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of assets at beginning of year $ 119,261    
Fair value of assets at end of year 140,164 $ 119,261  
Pension Plan [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 174,097 164,268  
Service cost 4,699 4,447 $ 5,072
Interest cost 6,993 6,830 6,342
Actuarial loss 16,552 3,967  
Implicit rate subsidy 0 0  
Benefits paid (6,134) (5,415)  
Benefit obligation at end of year 196,207 174,097 164,268
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of assets at beginning of year 113,895 104,953  
Actual return on plan assets 17,298 5,936  
Employer contributions 8,301 8,421  
Benefits paid (6,134) (5,415)  
Fair value of assets at end of year 133,360 113,895 104,953
Funded status at end of year (62,847) (60,202)  
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 16,461 16,420  
Service cost 529 527 379
Interest cost 634 655 504
Actuarial loss 1,124 (528)  
Implicit rate subsidy (196) (179)  
Benefits paid (549) (434)  
Benefit obligation at end of year 18,003 16,461 16,420
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of assets at beginning of year 5,366 4,462  
Actual return on plan assets 736 295  
Employer contributions 702 793  
Benefits paid 0 (184)  
Fair value of assets at end of year 6,804 5,366 $ 4,462
Funded status at end of year $ (11,199) $ (11,095)