Litigation and Related Matters (Narrative) (Details)	1 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended		36 Months Ended
	Sep. 30, 2010 site	Sep. 30, 2020 USD ($) Litigation	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Litigation site	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2007	Dec. 31, 2019 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									$ 39,800,000
Environmental liabilities		$ 1,500,000			$ 1,500,000		$ 1,600,000		1,600,000
Reserves for potential environmental liabilities		0	$ 0		0	$ 1,000,000.0
Settlement agreement amount paid for litigation agreement				$ 19,700,000
Accounts Payable and Accrued Expenses [Member]
Loss Contingencies [Line Items]
Environmental liabilities		$ 500,000			$ 500,000		100,000		100,000
St. Helens [Member]
Loss Contingencies [Line Items]
Environmental liabilities payment							$ 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%
Cost of Sales [Member]
Loss Contingencies [Line Items]
Settlement agreement amount of litigation agreement									9,200,000
SG&A Expenses [Member]
Loss Contingencies [Line Items]
Settlement agreement amount of litigation agreement					$ 200,000				$ 30,600,000