Contingencies - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended		9 Months Ended	12 Months Ended	24 Months Ended
	Sep. 30, 2018	Jul. 31, 2018	Sep. 30, 2018	Dec. 31, 2016	Dec. 31, 2016
Other Commitments [Line Items]
Administrative penalty	$ 1,600	$ 135
Consent Decree
Other Commitments [Line Items]
Civil penalty				$ 5,000	$ 83,000
Mitigation projects			$ 5,000
Supplemental environmental projects			$ 4,000
Administrative Penalty Subject To Deferral Period
Other Commitments [Line Items]
Administrative penalty		$ 41
Administrative Penalty Subject To Offset
Other Commitments [Line Items]
Administrative penalty	$ 1,400