Contingencies Remediation Liability (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Accrued Environmental Loss Contingencies, Current	$ 10,912	$ 11,274	$ 17,067
Accrual for Environmental Loss Contingencies, Charges to Expense for New Losses	234	1,182
Accrual for Environmental Loss Contingencies Payments, Net of Reimbursements	(594)	(6,967)
Accrual for Environmental Loss Contingencies, Foreign Currency Translation Gain (Loss)	$ (2)	$ (8)