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Certain Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]    
Projected benefit obligation $ 178,452 $ 170,716
Accumulated benefit obligation 178,452 170,716
Fair value of plan assets $ 163,460 $ 150,226