Nature of Operations (Schedule Of Property, Plant And Equipment Allocated To Geographic Area) (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	$ 630.7	$ 598.8
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	503.7	477.9
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	503.7	477.9
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	127.0	120.9
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	121.0	116.6
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant, and equipment, net	$ 6.0	$ 4.3