Contingencies (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2012
Site Contingency [Line Items]
Litigation Settlement, Amount		$ 110,000
Environmental Costs Recovered	$ 37,000
Huntsville Site [Member]
Site Contingency [Line Items]
Environmental Remediation Expense To Date	5,900
Additional Expected Project Expenditures Low Estimate	4,400
Additional Expected Project Expenditures High Estimate	10,000
Huntsville Site [Member] | Investigation Report [Member]
Site Contingency [Line Items]
Additional Expected Project Expenditures Low Estimate	300
Additional Expected Project Expenditures High Estimate	600
Norco Site [Member] | Remediation, Project Management, Regulatory Oversight, and Investigative and Feasability Studies [Member]
Site Contingency [Line Items]
Environmental Remediation Expense To Date	56,100
Additional Expected Project Expenditures Low Estimate	18,000
Additional Expected Project Expenditures High Estimate	$ 28,700