Contingencies Remediation Liability (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Cash Reimbursed for Environmental Liabilities	$ 811
Loss Contingencies [Line Items]
Accrued Environmental Loss Contingencies, Current	17,633	$ 18,176	$ 20,603
Accrual for Environmental Loss Contingencies, Charges to Expense for New Losses	299	556
Accrual for Environmental Loss Contingencies Payments, Net of Reimbursements	$ (842)	$ 2,983