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Employee Benefit Plans (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 25, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Cash payment for settlement of postretirement benefit plan obligation $ 31,616     $ 31,616  
Postretirement benefit plan obligation settlement interest 166        
Gain on settlement of postretirement benefit plan obligation, net of plaintiffs' attorneys' fees 14,306 $ 14,306   (14,306)  
Attorneys' fees 1,300        
Reduction in postretirement benefit obligation $ 68,806        
Defined contribution plan expense recognized     $ 1,486 2,062 $ 2,857
Minimum [Member]          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Contributions to pension plan     0    
Pension Benefits [Member]          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Cash payment for settlement of postretirement benefit plan obligation      
Defined benefit plan, future amortization of net gains (losses)     453    
Employer contributions      
Postretirement Benefit Plan [Member]          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Cash payment for settlement of postretirement benefit plan obligation     (31,616)  
Defined benefit plan, future amortization of net gains (losses)     (258)    
Defined Benefit Plan, Future Amortization of Prior Service Cost (Credit)     14    
Employer contributions     $ 539 $ 32,104