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Employee Benefit Plans Benefits (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Retirement plan and pension plan, total $ 12,289 $ 11,728 $ 10,525
Amount to be amortized from accumulated other comprehensive income (loss) in next twelve months 2,800    
Other Postretirement Benefit Plan      
Defined Benefit Plan Disclosure [Line Items]      
Pension Plan (497) (466) (52)
Plan amendment 2,500    
Other Postretirement Benefit Plan | 401k Plan      
Defined Benefit Plan Disclosure [Line Items]      
401(k) Retirement Plan $ 8,121 7,418 6,423
Maximum percentage of eligible employee’s covered compensation 5.00%    
Percentage of plan vested 100.00%    
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Pension Plan $ 4,168 $ 4,310 $ 4,102