Nature of Operations (Schedule Of Property, Plant And Equipment Allocated To Geographic Area) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	$ 566.4	$ 554.4
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	494.9	496.8
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	494.9	496.8
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	71.5	57.6
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	66.5	52.3
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	$ 5.0	$ 5.3