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Contingencies Remediation Liability (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Loss Contingencies [Line Items]      
Accrued Environmental Loss Contingencies, Current $ 14,570 $ 17,067 $ 18,176
Accrual for Environmental Loss Contingencies, Charges to Expense for New Losses 1,048 307  
Accrual for Environmental Loss Contingencies Payments, Net of Reimbursements $ (3,545) $ (1,416)