Nature of Operations (Schedule Of Property, Plant And Equipment Allocated To Geographic Area) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	$ 542.8	$ 529.9
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	490.6	486.0
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	490.6	486.0
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	52.2	43.9
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	46.5	38.2
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	$ 5.7	$ 5.7