Nature of Operations (Schedule Of Property, Plant And Equipment Allocated To Geographic Area) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	$ 554.4	$ 542.8
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	496.8	490.6
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	496.8	490.6
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	57.6	52.2
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	52.3	46.5
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	$ 5.3	$ 5.7