Acquisitions and Discontinued Operations (Narrative) (Details) $ in Millions				1 Months Ended		12 Months Ended
	Aug. 16, 2018 USD ($)	Jan. 13, 2017 USD ($)	Apr. 01, 2016	May 31, 2018 USD ($)	Apr. 30, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 31, 2018 USD ($)	Dec. 04, 2014 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, goodwill						$ 19.2	$ 11.3
(Loss) gain due to change in carrying value of discontinued operations net assets						(13.3)	(70.0)	$ 15.3
Separation costs								34.5
EMEA and Pacific Rim Business [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
(Loss) gain due to change in carrying value of discontinued operations net assets						(19.3)	(74.0)
EMEA and Pacific Rim Business [Member] | Foreign Currency Translation Adjustments [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
(Loss) gain due to change in carrying value of discontinued operations net assets						25.5	51.4
Flooring Businesses [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
(Loss) gain due to change in carrying value of discontinued operations net assets						$ 6.0	$ 4.0	15.3
Common stock split ratio of AFI share to AWI shares			0.5
Separation costs								$ 34.5
Initial term of campus lease agreement period						5 years
DLW [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Contingent liability										$ 12.9
Assets removed from balance sheet										151.9
Liabilities removed from balance sheet										$ 164.8
Settlement agreement and mutual release with administrator to settle all claims of Administrator related to insolvency for a cash payment					$ 11.8
Steel Ceilings, Inc. [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, purchase price	$ 12.3
Assets Held-for-sale, Not Part of Disposal Group, Current, Other									$ 2.0
Business acquisition, total fair value of tangible assets acquired, less liabilities assumed	4.4
Business acquisition, goodwill	3.2
Steel Ceilings, Inc. [Member] | Customer Relationships [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, total fair value of intangible assets acquired	1.4
Steel Ceilings, Inc. [Member] | TradeNames [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, total fair value of intangible assets acquired	$ 1.3
Plasterform, Inc. [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, purchase price				$ 11.9
Business acquisition, total fair value of tangible assets acquired, less liabilities assumed				2.2
Business acquisition, total fair value of intangible assets acquired				4.8
Business acquisition, goodwill				$ 4.9
Tectum [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, purchase price		$ 31.2
Business acquisition, total fair value of tangible assets acquired, less liabilities assumed		4.4
Business acquisition, total fair value of intangible assets acquired		16.0
Business acquisition, goodwill		$ 10.8