Contingencies Remediation Liability (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Accrued Environmental Loss Contingencies, Current	$ 15,119		$ 17,067	$ 18,176
Accrual for Environmental Loss Contingencies, Charges to Expense for New Losses	85	$ 307
Accrual for Environmental Loss Contingencies Payments, Net of Reimbursements	$ (2,033)	$ 1,416