Nature of Operations (Schedule Of Sales Allocated To Geographic Area) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	$ 936.9	$ 1,038.1	$ 975.3
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	726.0	826.6	801.6
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	674.1	769.0	739.2
Mineral Fiber [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	51.9	57.6	62.4
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	210.9	211.5	173.7
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	192.8	192.3	157.5
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	$ 18.1	$ 19.2	$ 16.2