Contingencies - Rollforward of Remediation Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Contingencies
Accrued liabilities start of period	$ 18,176	$ 20,603	$ 3,918
Remediation expense	307	556	18,591
Remediation payments	(1,416)	(2,983)	(1,906)
Accrued liabilities end of period	$ 17,067	$ 18,176	$ 20,603