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Employee Benefit Plans - Components of Expense (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]        
Net periodic benefit cost $ 3,411 $ 5,483 $ 7,639 $ 11,928
Pension and Other Postretirement Benefit Expense, Discontinued Operations 174 177 377 368
Pension and Other Postretirement Benefit Expense, Continuing Operations 3,237 5,306 7,262 11,560
Defined contribution plans        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]        
Net periodic benefit cost 1,978 1,957 4,771 4,861
Multi-employer plans        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]        
Net periodic benefit cost 44 59 91 133
Defined benefit plans        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]        
Interest cost 5,925 6,486 11,850 12,971
Expected return on plan assets, net of expenses (5,733) (4,360) (11,465) (8,720)
Amortization of actuarial loss 920 1,084 1,841 2,168
Total for defined benefit pension plans 1,112 3,210 2,226 6,419
SERPs        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]        
Net periodic benefit cost $ 277 $ 257 $ 551 $ 515