Litigation and Related Matters (Narrative) (Details)	1 Months Ended		3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2010 site	Sep. 30, 2018 USD ($) Litigation	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) Litigation site	Sep. 30, 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				2
Settlement agreement amount of litigation agreement	$ 6,500,000		$ 7,000,000						$ 30,500,000
Cash received from litigation settlements						$ 28,700,000
Environmental liabilities			13,300,000	$ 13,500,000			$ 13,300,000		$ 13,500,000
Reserves for potential environmental liabilities			500,000		$ 0		500,000	$ 1,300,000
Insurance recoveries related to environmental matters			7,000,000				$ 7,000,000
Litigation, defense cost			$ 2,700,000
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%