Nature of Operations (Schedule Of Sales Allocated To Geographic Area) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	$ 1,445.7	$ 1,295.2	$ 1,233.1
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	986.0	932.4	887.4
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	909.5	854.2	816.3
Mineral Fiber [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	76.5	78.2	71.1
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	459.7	362.8	345.7
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	448.8	349.3	322.1
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net sales	$ 10.9	$ 13.5	$ 23.6