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Pension and Other Retirement Plans - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Service cost $ 768 $ 692 $ 709
Interest cost 78,195 82,724 81,815
Expected return on plans’ assets (101,126) (107,616) (111,690)
Recognized actuarial loss 0 0 0
Amortization of prior service costs (credits) 116 90 0
Net periodic benefit (credit) cost (22,047) (24,110) (29,166)
Other Postretirement Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Service cost 0 0 81
Interest cost 264 320 451
Expected return on plans’ assets 0 0 0
Recognized actuarial loss 0 0 25
Amortization of prior service costs (credits) (380) (380) (81)
Net periodic benefit (credit) cost $ (116) $ (60) $ 476