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Retirement and Benefit Plans (Details) - USD ($)
$ in Thousands
 2 Months Ended 3 Months Ended 4 Months Ended 6 Months Ended
Aug. 03, 2018
Apr. 25, 2018
Jun. 30, 2018
Jun. 30, 2018
Jun. 30, 2017
Apr. 25, 2018
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]                  
Percentage of defined benefit plan assets transferred out of plan 40.00% 33.00%              
Net periodic benefit cost [Abstract]                  
Net periodic benefit expense             $ 13,026 $ 683  
Pension contributions [Abstract]                  
Pension contributions             1,042 1,145  
Pension Plan [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Increase (Decrease) for Assets Transferred to (from) Plan $ 122,000 $ 151,800              
Actuarial gain (loss)       $ 13,300          
Discount Rate   4.05%       4.05%     3.40%
Expected Long-term Rate of Return on Plan Assets     4.50%     4.75%      
Net periodic benefit cost [Abstract]                  
Service cost       198 $ 301   403 602  
Interest cost       3,312 4,382   7,323 8,758  
Expected return on plan assets       (3,292) (4,742)   (7,880) (9,482)  
Amortization of actuarial loss       356 410   1,177 805  
Plan settlement loss $ 10,000     12,003 0   12,003 0  
Net periodic benefit expense       12,577 $ 351   13,026 $ 683  
Pension contributions [Abstract]                  
Estimated remaining 2018 pension contributions     $ 21,000 21,000     21,000    
Discretionary estimated remaining 2018 pension contributions     $ 20,000 $ 20,000     $ 20,000