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Employee Benefit Plans: (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 15, 2017
Jul. 24, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Defined Benefit Pension Plans            
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]            
Service cost     $ 1,759 $ 2,078 $ 5,276 $ 6,234
Interest cost     3,880 3,936 11,640 11,808
Expected return on plan assets     (6,130) (5,766) (18,388) (17,297)
Prior service cost (benefit)     15 15 44 45
Net loss (gain)     1,002 1,793 3,005 5,379
Net periodic benefit cost     526 2,056 1,577 6,169
Pension and Other Postretirement Benefit Contributions [Abstract]            
Contributions by Employer $ 15,000 $ 13,000 27,700   27,700  
Estimated Future Employer Contributions in Current Fiscal Year         0  
Estimated Future Employer Contributions in Next Fiscal Year         12,700  
Defined Benefit Postretirement Healthcare Plans            
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]            
Service cost     575 467 1,725 1,401
Interest cost     533 485 1,600 1,455
Expected return on plan assets     (79) (70) (237) (210)
Prior service cost (benefit)     (109) (107) (327) (321)
Net loss (gain)     125 84 375 252
Net periodic benefit cost     1,045 859 3,136 2,577
Pension and Other Postretirement Benefit Contributions [Abstract]            
Contributions by Employer     1,270   3,810  
Estimated Future Employer Contributions in Current Fiscal Year         1,270  
Estimated Future Employer Contributions in Next Fiscal Year         5,115  
Supplemental Non-qualified Defined Benefit and Defined Contribution Plans            
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]            
Service cost     612 623 2,048 1,530
Interest cost     319 314 957 943
Prior service cost (benefit)     0 1 1 2
Net loss (gain)     251 207 751 621
Net periodic benefit cost     1,182 $ 1,145 3,757 $ 3,096
Pension and Other Postretirement Benefit Contributions [Abstract]            
Contributions by Employer     $ 395   1,187  
Estimated Future Employer Contributions in Current Fiscal Year         396  
Estimated Future Employer Contributions in Next Fiscal Year         $ 1,682