Nature of Operations (Schedule Of Sales Allocated To Geographic Area) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	$ 1,295.2	$ 1,233.1	$ 1,106.6
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	932.4	887.4	818.5
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	854.2	816.3	754.2
Mineral Fiber [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	78.2	71.1	64.3
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	362.8	345.7	288.1
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	349.3	322.1	268.0
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	$ 13.5	$ 23.6	$ 20.1