Nature of Operations (Schedule Of Sales Allocated To Geographic Area) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	$ 1,106.6	$ 936.9	$ 1,038.1
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	818.5	726.0	826.6
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	754.2	674.1	769.0
Mineral Fiber [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	64.3	51.9	57.6
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	288.1	210.9	211.5
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	268.0	192.8	192.3
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	$ 20.1	$ 18.1	$ 19.2