Litigation and Related Matters (Narrative) (Details)	1 Months Ended	3 Months Ended			12 Months Ended
	Sep. 30, 2010 site	Mar. 31, 2018 USD ($) Litigation site	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2007	Feb. 01, 2018 USD ($)
Loss Contingencies [Line Items]
Number of active and independent litigation matters for which pursuing coverage and recoveries | Litigation		2
Settlement agreement amount of litigation agreement					$ 30,500,000
Cash received from litigation settlements		$ 28,700,000
Environmental liabilities		13,200,000	$ 13,500,000		$ 13,500,000
Reserves for potential environmental liabilities		$ 0		$ 600,000
St. Helens [Member]
Loss Contingencies [Line Items]
Environmental liabilities							$ 8,600,000
Reserves for potential environmental liabilities			$ 8,600,000
Macon Site [Member]
Loss Contingencies [Line Items]
Number of landfills listed as Superfund site | site		2
Number of landfills AWI entered into an Administrative Order on Consent for a Removal Action | site	1
Submission date of final report to EPA	Oct. 11, 2016
Elizabeth City [Member]
Loss Contingencies [Line Items]
Percentage of site costs Navy agreed to pay						33.33%