Nature of Operations (Schedule Of Sales Allocated To Geographic Area) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	$ 1,038.1	$ 975.3	$ 893.6
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	826.6	801.6	756.4
Mineral Fiber [Member] | United States and Latin America [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	769.0	739.2	699.8
Mineral Fiber [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	57.6	62.4	56.6
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	211.5	173.7	137.2
Architectural Specialties [Member] | United States and Latin America [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	192.3	157.5	129.5
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	$ 19.2	$ 16.2	$ 7.7