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Benefit Plans Reconciliation of Funded Status (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of assets at beginning of year $ 109,415    
Fair value of assets at end of year 119,261 $ 109,415  
Pension Plan [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 164,268 165,304  
Service cost 4,447 5,072 $ 3,938
Interest cost 6,830 6,342 6,098
Actuarial loss 3,967 (7,436)  
Implicit rate subsidy 0 0  
Benefits paid (5,415) (5,014)  
Benefit obligation at end of year 174,097 164,268 165,304
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of assets at beginning of year 104,953 99,628  
Actual return on plan assets 5,936 965  
Employer contributions 8,421 9,374  
Benefits paid (5,415) (5,014)  
Fair value of assets at end of year 113,895 104,953 99,628
Funded status at end of year (60,202) (59,315)  
Other Postretirement Benefit Plan [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 16,420 13,714  
Service cost 527 379 284
Interest cost 655 504 508
Actuarial loss (528) 2,231  
Implicit rate subsidy (179) 0  
Benefits paid (434) (408)  
Benefit obligation at end of year 16,461 16,420 13,714
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of assets at beginning of year 4,462 4,251  
Actual return on plan assets 295 79  
Employer contributions 793 468  
Benefits paid (184) (336)  
Fair value of assets at end of year 5,366 4,462 $ 4,251
Funded status at end of year $ (11,095) $ (11,958)