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Pension Plans - Multi-employer (Details) - Multi-employer defined benefit pension plan - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Jun. 30, 2012
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Multiemployer Plans [Line Items]          
Insolvent period   20 years      
Estimated liability upon withdrawal from plan     $ 81,600 $ 82,200 $ 72,700
Percentage of annual compounded surcharge for rehabilitation   5.00%      
Percentage of interim surcharge 5.00%        
Pension expense     2,836 2,617  
Increase in surcharge     $ 811 $ 656