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Nature of Operations (Schedule Of Property, Plant And Equipment Allocated To Geographic Area) (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Revenues From External Customers And Long Lived Assets [Line Items]    
Property, plant, and equipment, net $ 598.8 $ 566.4
Mineral Fiber [Member]    
Revenues From External Customers And Long Lived Assets [Line Items]    
Property, plant, and equipment, net 477.9 494.9
Mineral Fiber [Member] | United States [Member]    
Revenues From External Customers And Long Lived Assets [Line Items]    
Property, plant, and equipment, net 477.9 494.9
Architectural Specialties [Member]    
Revenues From External Customers And Long Lived Assets [Line Items]    
Property, plant, and equipment, net 120.9 71.5
Architectural Specialties [Member] | United States [Member]    
Revenues From External Customers And Long Lived Assets [Line Items]    
Property, plant, and equipment, net 116.6 66.5
Architectural Specialties [Member] | Canada [Member]    
Revenues From External Customers And Long Lived Assets [Line Items]    
Property, plant, and equipment, net $ 4.3 $ 5.0